The Collaborative International Dictionary of English v.0.48:

Dog days \Dog" days`\, dog-days \dog-days\
   A period of from four to six weeks, in the summer, variously
   placed by almanac makers between the early part of July and
   the early part of September; canicular days; -- so called in
   reference to the rising in ancient times of the Dog Star
   (Sirius) with the sun. Popularly, the sultry, close part of
   the summer; metaphorically, a period of inactivity.

   Syn: dog days, canicule, canicular days.
        [1913 Webster + WordNet 1.5]

   Note: The conjunction of the rising of the Dog Star with the
         rising of the sun was regarded by the ancients as one
         of the causes of the sultry heat of summer, and of the
         maladies which then prevailed. But as the conjunction
         does not occur at the same time in all latitudes, and
         is not constant in the same region for a long period,
         there has been much variation in calendars regarding
         the limits of the dog days. The astronomer Roger Long
         states that in an ancient calendar in Bede (died 735)
         the beginning of dog days is placed on the 14th of
         July; that in a calendar prefixed to the Common Prayer,
         printed in the time of Queen Elizabeth, they were said
         to begin on the 6th of July and end on the 5th of
         September; that, from the Restoration (1660) to the
         beginning of New Style (1752), British almanacs placed
         the beginning on the 19th of July and the end on the
         28th of August; and that after 1752 the beginning was
         put on the 30th of July, the end on the 7th of
         September. Some English calendars now put the beginning
         on July 3d, and the ending on August 11th. A popular
         American almanac of the present time (1890) places the
         beginning on the 25th of July, and the end on the 5th
         of September.
         [1913 Webster]

WordNet (r) 3.0 (2006):

dog days
    n 1: the hot period between early July and early September; a
         period of inactivity [syn: dog days, canicule,
         canicular days]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "dog days":
   broiling sun, canicular days, fair weather, good old summertime,
   growing season, heat wave, high summer, hot wave, hot weather,
   humid weather, midday sun, midsummer, muggy weather,
   stuffy weather, sultry weather, summer, summertide, summertime,
   sunshiny weather, vertical rays, warm weather