The term 'calendar period' encompasses a variety of different time frames. The five solutions to the crossword are EPACT (5 letters), WEEK (4 letters), MONTH (5 letters), YEAR (4 letters), and DAY (3 letters). An epact is a number that is used to calculate the date of Easter in the Christian calendar. A WEEK is a period of seven days, usually beginning on Sunday. A MONTH is a period of time that is typically one-twelfth of a year, lasting between 28 and 31 days. A year is a period of 365 or 366 days, depending on whether it is a leap year. Finally, a DAY is a period of 24 hours.