Here's a pop quiz. Think fast. What date does Christmas fall on every year? Valentine's Day? St. Patrick's Day? Halloween? What about Easter?

If you were on a roll until that last question, you're not alone, and it's not your fault. You probably already know that Easter falls on a different date every year. But why?

Easter and the many church holidays related to it - such as Ash Wednesday, Palm Sunday, Good Friday - are called "moveable feasts," because they do not fall on a fixed date on the Gregorian calendar, which follows the cycle of the sun and the seasons. Instead, these days follow a lunisolar calendar, similar to Jewish holidays.

According to a Fourth Century ruling, the date of Easter is set for on the first Sunday following the Paschal Full Moon, which is the first Full Moon of Spring, occurring on or shortly after the Vernal Equinox. March 22 is the earliest Easter can occur on any given year, and April 25 is the latest.

Some churches, primarily Eastern Orthodox Churches, use the older Julian calendar for holidays. Easter sometimes falls on different dates, according to that calendar.

Here's a look at when Easter will be celebrated over the next decade.

2010    April 4

2011    April 24

2012    April 8

2013    March 31

2014    April 20

2015    April 5

2016    March 27

2017    April 16

2018    April 1

2019    April 21

2020    April 12