When is Passover this year?
Passover begins at sundown on Friday, April 15 and ends at sundown on Saturday, April 23. Most people host seders on the first two nights of the holiday. There is also a tradition in many North African Jewish communities of celebrating Mimouna after the end of Passover. Families gather and mark the return to eating chametz (leavened bread).
Why does Passover arrive on different dates every year?
It can be a bit confusing trying to figure out the Jewish calendar for holidays, until you realize that Jews, in fact, use a different calendar than the Gregorian one used throughout most of the world, though not exclusively. Essentially, Jews double dip, using the Gregorian calendar for secular affairs and sticking to the Jewish one for all dates related to Jewish holidays and other religious events, as well as national Israeli holidays.
To clarify, the Jewish calendar is lunar rather than solar, not an uncommon phenomenon in the context of the ancient world. In Biblical times, in fact, the start of each new month had to be officially determined in the Sanhedrin, or Jewish high court, in Jerusalem based on the testimony of witnesses who could give testimony of having observed the new, crescent moon (coinciding with what Jewish astronomers knew would be the right date). These witnesses were even cross-examined to make sure they had, indeed, seen the new moon, meaning a new month could be officially declared and sanctified! In those days, the message would be spread from Jerusalem outwards. This is why for the majority of Jewish holidays, two days are celebrated in the Diaspora where only one is celebrated within Israel. This practice was instituted by Diaspora Jews so they would not unwittingly miss the proper day to celebrate holidays should the declaration of the new month tarry in reaching them.
Beginning in the 4th century, things were standardized as Jewish communities spread throughout the world and the Sanhedrin was eventually disbanded. Since then, Jews have been using a fixed calendar, but the moon is still the determining factor. There are 12 lunar months per year of 29 or 30 days each, totaling 354 days in the lunar calendar. To keep the lunar calendar from shifting out of sync with the solar calendar, however, leap years occur few years to accommodate for the discrepancy.
So how do we decide when Passover is exactly?
Passover has the special honor of falling in the Hebrew month of Nissan (no relation to the automobile make), the start of the Jewish calendar marking spring and the beginning of the agricultural cycle in Israel. While the Jewish New Year, or Rosh ha-Shanah, falls in Tishrei, this is actually considered the seventh month of the Jewish calendar. This may sound confusing, but actually Passover is one of the four Jewish new years, specifically the New Year of the creation of the Jewish people, as the Exodus from Egypt and the subsequent reception of the Torah at Mt. Sinai are the most formative experiences in the collective Jewish narrative.