Special days and periods (such as leap day or intercalary day). Alignment with social cycles. Alignment with astronomical cycles. Alignment with biological cycles. Literal notation of dates. Historical reforms Historically, most calendar reforms have been made in order to synchronize the calendar with the astronomical year (either solar or sidereal) and/or the synodic month in lunar or lunisolar calendars. Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar when it was altered to the Gregorian calendar. The fundamental problems of the calendar are that the astronomical year has neither a whole number of days nor a whole number of lunar months; neither does the lunar month have a whole number of days: in each case there are fractions "left over". (In some physical circumstances the rotations and revolutions of a planet and its satellite(s) can be phase-locked —for example the same side of the moon always faces us— but this has not operated to lock together the lengths of the Earth's year, day and month.) Such remainders could accumulate from one period to the next, thereby driving the cycles out of synchronization. A typical solution to force synchronization is intercalation. This means occasionally adding an extra day (or month) into the cycle. An alternative approach is to ignore the mismatch and simply let the cycles continue to drift apart. The general approaches include: The lunar calendar, which fits days into the cycle of lunar months, adding an extra day when needed, while ignoring the annual solar cycle of the seasons. The solar calendar, which fits artificial months into the year, adding an extra day into one month when needed, while ignoring the lunar cycle of new/full moons. The lunisolar calendar, which keeps both the lunar and solar cycles, adding an extra month into the year when needed. An obvious disadvantage of the lunisolar method of inserting a whole extra month is the large irregularity of the length of the year from one to the next. The simplicity of a lunar calendar has always been outweighed by its inability to track the seasons, and a solar calendar is used in conjunction to remedy this defect. Identifying the lunar cycle month requires straightforward observation of the Moon on a clear night. However, identifying seasonal cycles requires much more methodical observation of stars or a device to