day of the week and calendars expire every year. It is difficult to determine the weekday of any given day of the year or month. Months are not equal in length, nor regularly distributed across the year, and so some people rely on mnemonics (e.g., "Thirty days hath September" or knuckle counting) to remember the lengths of months. The year's four quarters (of three full months each) are not equal (being of 90/91, 91, 92 and 92 days respectively). Business quarters that are equal would make accounting easier. Its epoch, i.e. start of the year count, is religious. The same applies to month and weekday names in many languages. Each month has no connection with the lunar phases. Solstices and equinoxes do not coincide with either the beginning of the Gregorian months or the midpoint of the months. The calendar does not have a year zero: the year after 1 BC was 1 AD, with nothing in between them. It is hard or even impossible to solve all these issues in just one calendar. Most plans evolve around the solar year of a little more than 365 days. This number does not divide well by seven or twelve, which are the traditional numbers of days per week and months per year respectively. The nearby numbers 360, 364 and 366 are divisible in better ways. There are also lunar-centric proposals. Perennial calendars 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. Proposals mainly differ in their selection of a leap rule, placing of the leap item (usually middle or end of the year), in the start day of the week and year, in the number (12 or 13) and size of months and in connected naming; some are compatible to the week date of ISO 8601. The World Calendar, favored by the UN in the 1950s, and the