Wiktionary
n. 1 (context legal English) A timespan of exactly 24 hours, measured from one midnight to the next. 2 A day having a conventional designation on a recognized calendar, such as a numerical identification within a named month.
WordNet
n. a day reckoned from midnight to midnight [syn: civil day]
Wikipedia
A calendar day is any day of the week, including weekends. It relates to any day of the week, month or year.
It contrasts the weekday, which usually means any day of the week except the weekend days, which in most parts of the world are Saturday, Sunday or both. Consequently, two weeks may consist of 10 or 12 weekdays depending on local usage of the term weekday, but they always consist of 14 calendar days. The term calendar day is useful in situations where "weekday" can be mistaken to mean any day of the week.
The calendar day also contrasts the working or business day, which excludes holidays of all sorts, but in some cultures may include one of the weekend days, e.g. Saturday, although it is not considered a weekday. Also, a calendar day always runs 24 hours (except for DST changes) from 00:00 through 24:00, whereas a business day may be constrained by work or opening hours and may span over midnight.
The term "calendar day" usually denotes a time span, unlike calendar date which can denote a point in time.
Usage examples of "calendar day".
The calendar day that comes between the end of the old year and the start of the new, the day when the lid's off.
But if you prefer, you can, as some modern practitioners do, count the Dark of the Moon only as those three last waning days and start new moon magick from the literal calendar day.
One Crocodile, meaning the first time in the calendar day crocodile occurred, was considered a fortuitous day to be born on.
The most successful had been the one named Montag--amusing to be named for a calendar day-but even Montag had reached his limit well below adept.