The holiday of Shavuot, the Festival of Weeks, falls this year on Wednesday in Israel and on Wednesday and Thursday in the Diaspora. It falls every year exactly seven weeks after Passover. The latter holiday celebrates the Israelites’ liberation from slavery in Egypt; Shavuot (which means “weeks”) celebrates the giving of the Torah on Mount Sinai, which followed some arduous trekking through the desert.
Shavuot, though, has a whole other, agricultural dimension. Also known as the Festival of the First Fruits, in ancient Israel Shavuot marked the end of the spring barley harvest and the bringing of the first fruits to the Temple in Jerusalem. Theorists of these matters believe the agricultural layer of the holiday is the older, original one, and the commemoration of Sinai was added later.
In any case, the Sinai dimension of the holiday is more portable and can be practiced in synagogues anywhere; the agricultural dimension is more tied to the land of Israel. In fact, growing up in a secular Jewish family in upstate New York, I didn’t know about Shavuot at all. We had a Passover meal every year, and I thought it pretty much ended with that.
It makes sense, then, that during the period of Zionist resettlement of the land of Israel, the agricultural aspect was intensely revived. In fact, it was revived particularly by the kibbutz movements — which, at the time, were doctrinally socialist and mostly atheist, but seeking roots in the soil of the land.