Page "Calendar" Paragraph 23

Solar calendars assign a date to each solar day.

A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets.

The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day.

Other types of calendar may also use a solar day.