from
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]