This week is a week of holy days for people of the Jewish and Christian faiths. For Judaism, the Passover celebration began last night; for Christians (both western and eastern), Easter is this Sunday. The exact days change from year to year. Why?
The Jewish calendar is based to a large extent on the moon. The moon completes a cycle of phases every 29.5 days; twelve "moonths" (or lunar months) add up to a bit over 354 days. Note that this is 11 days short of a normal, or "solar" year. So, the Passover celebration, which occurs in the Jewish religious month of Nissan, can shift with regard to the Gregorian calendar that we all use. Every few years, an extra month has to be added to the calendar to keep the lunar calendar in line with the sun, so that the Passover celebration always occurs in the spring.
The date of Easter used to be fixed to the date of Passover, since the Christian holiday is based on events that occurred during the Passover celebration. More recently, the date was changed so that Easter is the first Sunday following the first full moon of spring. In western Christianity, most years the two holidays coincide. In eastern (orthodox) Christianity, due to vagaries of different calendars used, the Easter celebration is often a lunar cycle later (though this year they coincide).
There are many other calendars based on the lunar cycle. The Islamic calendar sticks very precisely to the lunar cycle, so dates of Islamic holidays drift with respect to the seasons. Most other lunar calendars have an extra month added in now and then to try and keep track with the seasons.
Calendars are all, to some extent, artificial. There are not an even number of days in a lunar cycle (or in a year, which is why we need leap days now and then), there are not an equal number of lunar cycles in a year. But for the purposes of time keeping, a day, a lunar cycle, and a year are natural divisions of time if you don't have watches and calendars. In fact, in many ways the Gregorian calendar that we use is even more arbitrary -- a month doesn't follow the lunar cycle, and the lengths of the months vary from 28 to 31 days in an irregular fashion, and every 4th year (except every 100th year, except once again for every 400th year) we add an extra day to our calendar.
If you are celebrating holy days this week, a happy Passover or happy Easter to you all. And, even if you aren't celebrating, you know that the full moon is this week, so go out in the evening and enjoy the big yellow moon.