The Collaborative International Dictionary of English v.0.48:

Day \Day\ (d[=a]), n. [OE. day, dai, dei, AS. d[ae]g; akin to
   OS., D., Dan., & Sw. dag, G. tag, Icel. dagr, Goth. dags; cf.
   Skr. dah (for dhagh ?) to burn. [root]69. Cf. Dawn.]
   1. The time of light, or interval between one night and the
      next; the time between sunrise and sunset, or from dawn to
      darkness; hence, the light; sunshine; -- also called
      daytime.
      [1913 Webster +PJC]

   2. The period of the earth's revolution on its axis. --
      ordinarily divided into twenty-four hours. It is measured
      by the interval between two successive transits of a
      celestial body over the same meridian, and takes a
      specific name from that of the body. Thus, if this is the
      sun, the day (the interval between two successive transits
      of the sun's center over the same meridian) is called a
      solar day; if it is a star, a sidereal day; if it is
      the moon, a lunar day. See Civil day, Sidereal day,
      below.
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   3. Those hours, or the daily recurring period, allotted by
      usage or law for work.
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   4. A specified time or period; time, considered with
      reference to the existence or prominence of a person or
      thing; age; time.
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            A man who was great among the Hellenes of his day.
                                                  --Jowett
                                                  (Thucyd. )
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            If my debtors do not keep their day, . . .
            I must with patience all the terms attend. --Dryden.
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   5. (Preceded by the) Some day in particular, as some day of
      contest, some anniversary, etc.
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            The field of Agincourt,
            Fought on the day of Crispin Crispianus. --Shak.
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            His name struck fear, his conduct won the day.
                                                  --Roscommon.
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   Note: Day is much used in self-explaining compounds; as,
         daybreak, daylight, workday, etc.
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   Anniversary day. See Anniversary, n.

   Astronomical day, a period equal to the mean solar day, but
      beginning at noon instead of at midnight, its twenty-four
      hours being numbered from 1 to 24; also, the sidereal day,
      as that most used by astronomers.

   Born days. See under Born.

   Canicular days. See Dog day.

   Civil day, the mean solar day, used in the ordinary
      reckoning of time, and among most modern nations beginning
      at mean midnight; its hours are usually numbered in two
      series, each from 1 to 12. This is the period recognized
      by courts as constituting a day. The Babylonians and
      Hindoos began their day at sunrise, the Athenians and Jews
      at sunset, the ancient Egyptians and Romans at midnight.


   Day blindness. (Med.) See Nyctalopia.

   Day by day, or Day after day, daily; every day;
      continually; without intermission of a day. See under
      By. "Day by day we magnify thee." --Book of Common
      Prayer.

   Days in bank (Eng. Law), certain stated days for the return
      of writs and the appearance of parties; -- so called
      because originally peculiar to the Court of Common Bench,
      or Bench (bank) as it was formerly termed. --Burrill.

   Day in court, a day for the appearance of parties in a
      suit.

   Days of devotion (R. C. Ch.), certain festivals on which
      devotion leads the faithful to attend mass. --Shipley.

   Days of grace. See Grace.

   Days of obligation (R. C. Ch.), festival days when it is
      obligatory on the faithful to attend Mass. --Shipley.

   Day owl, (Zool.), an owl that flies by day. See Hawk owl.


   Day rule (Eng. Law), an order of court (now abolished)
      allowing a prisoner, under certain circumstances, to go
      beyond the prison limits for a single day.

   Day school, one which the pupils attend only in daytime, in
      distinction from a boarding school.

   Day sight. (Med.) See Hemeralopia.

   Day's work (Naut.), the account or reckoning of a ship's
      course for twenty-four hours, from noon to noon.

   From day to day, as time passes; in the course of time; as,
      he improves from day to day.

   Jewish day, the time between sunset and sunset.

   Mean solar day (Astron.), the mean or average of all the
      apparent solar days of the year.

   One day, One of these days, at an uncertain time, usually
      of the future, rarely of the past; sooner or later. "Well,
      niece, I hope to see you one day fitted with a husband."
      --Shak.

   Only from day to day, without certainty of continuance;
      temporarily. --Bacon.

   Sidereal day, the interval between two successive transits
      of the first point of Aries over the same meridian. The
      Sidereal day is 23 h. 56 m. 4.09 s. of mean solar time.

   To win the day, to gain the victory, to be successful. --S.
      Butler.

   Week day, any day of the week except Sunday; a working day.


   Working day.
      (a) A day when work may be legally done, in distinction
          from Sundays and legal holidays.
      (b) The number of hours, determined by law or custom,
          during which a workman, hired at a stated price per
          day, must work to be entitled to a day's pay.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

By \By\ (b[imac]), prep. [OE. bi, AS. b[imac], big, near to, by,
   of, from, after, according to; akin to OS. & OFries. bi, be,
   D. bij, OHG. b[imac], G. bei, Goth. bi, and perh. Gr. 'amfi`.
   E. prefix be- is orig. the same word. [root]203. See pref.
   Be-.]
   1. In the neighborhood of; near or next to; not far from;
      close to; along with; as, come and sit by me.
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            By foundation or by shady rivulet
            He sought them both.                  --Milton.
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   2. On; along; in traversing. Compare 5.
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            Long labors both by sea and land he bore. --Dryden.
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            By land, by water, they renew the charge. --Pope.
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   3. Near to, while passing; hence, from one to the other side
      of; past; as, to go by a church.
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   4. Used in specifying adjacent dimensions; as, a cabin twenty
      feet by forty.
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   5. Against. [Obs.] --Tyndale [1. Cor. iv. 4].
      [1913 Webster]

   6. With, as means, way, process, etc.; through means of; with
      aid of; through; through the act or agency of; as, a city
      is destroyed by fire; profit is made by commerce; to take
      by force.
      [1913 Webster]

   Note: To the meaning of by, as denoting means or agency,
         belong, more or less closely, most of the following
         uses of the word:
      (a) It points out the author and producer; as, "Waverley",
          a novel by Sir W.Scott; a statue by Canova; a sonata
          by Beethoven.
      (b) In an oath or adjuration, it indicates the being or
          thing appealed to as sanction; as, I affirm to you by
          all that is sacred; he swears by his faith as a
          Christian; no, by Heaven.
      (c) According to; by direction, authority, or example of;
          after; -- in such phrases as, it appears by his
          account; ten o'clock by my watch; to live by rule; a
          model to build by.
      (d) At the rate of; according to the ratio or proportion
          of; in the measure or quantity of; as, to sell cloth
          by the yard, milk by the quart, eggs by the dozen,
          meat by the pound; to board by the year.
      (e) In comparison, it denotes the measure of excess or
          deficiency; when anything is increased or diminished,
          it indicates the measure of increase or diminution;
          as, larger by a half; older by five years; to lessen
          by a third.
      (f) It expresses continuance or duration; during the
          course of; within the period of; as, by day, by night.
      (g) As soon as; not later than; near or at; -- used in
          expressions of time; as, by this time the sun had
          risen; he will be here by two o'clock.
          [1913 Webster]

   Note: In boxing the compass, by indicates a pint nearer to,
         or towards, the next cardinal point; as, north by east,
         i.e., a point towards the east from the north;
         northeast by east, i.e., on point nearer the east than
         northeast is.
         [1913 Webster]

   Note: With is used instead of by before the instrument with
         which anything is done; as, to beat one with a stick;
         the board was fastened by the carpenter with nails. But
         there are many words which may be regarded as means or
         processes, or, figuratively, as instruments; and
         whether with or by shall be used with them is a matter
         of arbitrary, and often, of unsettled usage; as, to a
         reduce a town by famine; to consume stubble with fire;
         he gained his purpose by flattery; he entertained them
         with a story; he distressed us with or by a recital of
         his sufferings. see With.
         [1913 Webster]

   By all means, most assuredly; without fail; certainly.

   By and by.
      (a) Close together (of place). [Obs.] "Two yonge knightes
          liggyng [lying] by and by." --Chaucer.
      (b) Immediately; at once. [Obs.] "When . . . persecution
          ariseth because of the word, by and by he is
          offended." --Matt. xiii. 21.
      (c) Presently; pretty soon; before long.

   Note: In this phrase, by seems to be used in the sense of
         nearness in time, and to be repeated for the sake of
         emphasis, and thus to be equivalent to "soon, and
         soon," that is instantly; hence, -- less emphatically,
         -- pretty soon, presently.

   By one's self, with only one's self near; alone; solitary.

   By the bye. See under Bye.

   By the head (Naut.), having the bows lower than the stern;
      -- said of a vessel when her head is lower in the water
      than her stern. If her stern is lower, she is by the
      stern.

   By the lee, the situation of a vessel, going free, when she
      has fallen off so much as to bring the wind round her
      stern, and to take her sails aback on the other side.

   By the run, to let go by the run, to let go altogether,
      instead of slacking off.

   By the way, by the bye; -- used to introduce an incidental
      or secondary remark or subject.

   Day by day, One by one, Piece by piece, etc., each day,
      each one, each piece, etc., by itself singly or
      separately; each severally.

   To come by, to get possession of; to obtain.

   To do by, to treat, to behave toward.

   To set by, to value, to esteem.

   To stand by, to aid, to support.
      [1913 Webster]

   Note: The common phrase good-by is equivalent to farewell,
         and would be better written good-bye, as it is a
         corruption of God be with you (b'w'ye).
         [1913 Webster]

WordNet (r) 3.0 (2006):

day by day
    adv 1: gradually and progressively; "his health weakened day by
           day" [syn: day by day, daily]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "day by day":
   again and again, cyclically, daily, day after day, epochally,
   every day, every hour, frequently, hour by hour, hourly, in time,
   many a time, many times, many times over, often, on the beat,
   over and over, periodically, recurrently, repeatedly, rhythmically,
   seasonally, several times, synchronously, time after time,
   time and again, times without number, year after year