A calendar day is a period from midnight on a given day to midnight on the next day. Thus, a calendar day is a period of 24 hours starting from midnight.
A calendar day is different from a weekday as it includes only five working days of the week. A Saturday and a Sunday are both calendar days, but they are not week days. These are called weekend days.
It is different from a business day or a working day. A business day starts when work starts in offices on a given day. Typically, a business day is from 9 a.m to 5 p.m. A week has seven calendar days. There are 30 or 31 calendars in a month, except for February, which has 28 or 29 calendar days.