Deuternomy 16:1

שָׁמוֹר֙ אֶת־חֹ֣דֶשׁ הָאָבִ֔יב וְעָשִׂ֣יתָ פֶּ֔סַח לַיהֹוָ֖ה אֱלֹהֶ֑יךָ כִּ֞י בְּחֹ֣דֶשׁ הָֽאָבִ֗יב הוֹצִ֨יאֲךָ֜ יְהֹוָ֧ה אֱלֹהֶ֛יךָ מִמִּצְרַ֖יִם לָֽיְלָה׃

Observe the month of Aviv and offer a passover sacrifice to the Lord your God, for it was in the month of Aviv, at night, that the Lord your God freed you from Egypt.

It’s amazing how one verse, one word even can change so much. In this case, the entire way the Jewish people tell time, the way our calendar functions was determined by one word in this verse. Our calendar could have gone in two other directions. Like the calendar of the Christian world, we could have measured our months and years based on the sun alone, a solar year, 365 days. Like the Muslim world, we could have measured our calendar by the cycles of the moon. This would have given us a slightly shorter year of 354 days. Instead, the Jewish calendar is both. 

Our calendar is lunar, in that our months begin with the new moon. Our holidays fall on specific days of our lunar months, which is why they seem to be “late” or “early” in any given year. Lunar months shift relative to the solar year. Our calendar is also measured by the rhythm of the sun. Passover, though falling in the lunar month of Nissan, must also fall in the springtime, usually March 20th. For this reason we have Jewish leap years where we add a whole extra month in case the holidays fall too early, and we have to bump the holidays back to their proper season.

If all of this is confusing, you can blame it on this verse. “Observe the month of Aviv” the Torah says in regards to Passover. Aviv means Springtime, and because of this verse, because of that word, Passover must fall in the Spring, as well as on the 15th of the lunar month of Nisan. That one word causes our calendar an added complication of not being simply lunar or solar, but requiring us to use both of these mysterious luminaries to set our clocks.