What is Calendar Day?

Legal Definition
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.
-- Wikipedia
Legal Definition
So many days reckoned according to the course of the calendar. For example, a note dated January 1st and payable "thirty calendar days after date," without grace, is payable on the 31st day of January, though if expressed to be payable simply "thirty days after date," it would be payable February 1st.
-- Black's Law Dictionary