The Collaborative International Dictionary of English v.0.48:

Almanac \Al"ma*nac\ (?; 277), n. [LL. almanac, almanach: cf. F.
   almanach, Sp. almanaque, It. almanacco, all of uncertain
   origin.]
   A book or table, containing a calendar of days, and months,
   to which astronomical data and various statistics are often
   added, such as the times of the rising and setting of the sun
   and moon, eclipses, hours of full tide, stated festivals of
   churches, terms of courts, etc.
   [1913 Webster]

   Nautical almanac, an almanac, or year book, containing
      astronomical calculations (lunar, stellar, etc.), and
      other information useful to mariners.
      [1913 Webster]

WordNet (r) 3.0 (2006):

almanac
    n 1: an annual publication including weather forecasts and other
         miscellaneous information arranged according to the
         calendar of a given year [syn: almanac, farmer's
         calendar]
    2: an annual publication containing tabular information in a
       particular field or fields arranged according to the calendar
       of a given year

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ALMANAC. A table or calendar, in which are set down the revolutions of the
seasons, the rising and setting of the sun, the phases of the moon, the most
remarkable conjunctions, positions and phenomena of the heavenly bodies, the
months of the year, the days of the month and week, and a variety of other
matter.
     2. The courts will take judicial notice of the almanac; for example,
whether a certain day of the month was on a Sunday or not. Vin. Ab. h.t.; 6
Mod. 41; Cro. Eliz. 227, pl. 12; 12 Vin. Ab. Evidence (A, b, 4.) In dating
instruments, some sects, the Quakers, for example, instead of writing
January, February, March, &c., use the terms, First month, Second month,
Third month, &c., and these are equally valid in such writings. Vide 1
Smith's Laws of Pennsylvania, 217.