Many calendar reforms have offered solutions to make the Gregorian calendar perennial. These reforms would make it easy to work out the day of the week of a particular date, and would make changing calendars each year unnecessary. There are, roughly speaking, two options to achieve this goal: leap week calendars and intercalary days. Leap week calendars add a leap week of seven days to the calendar every five or six years to keep the calendar roughly in step with the tropical year. They have years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week.