Yom HaAliyah is an Israeli national holiday. It is celebrated both on 10 Nisan and on 7 Heshvan—the latter being the school celebration, occurring this coming Monday evening through Tuesday. The observance commemorates the historic events of 10 Nisan, and thus acknowledges aliyah, immigration to the Jewish state, as a core value of the country. It also honors the contributions of immigrants to Israeli society. Because the 10th of Nisan occurs a few days before Passover, when schools are not in session, the observance is moved to a time when schools are open.
The celebration began in 2012 as a grassroots initiative in Tel Aviv under the auspices of the Am Yisrael Foundation. On June 21, 2016, the Knesset voted in favor of codifying the initiative, and adding it to the official national calendar.
So what happened in history on 10 Nisan? On that day, according to the Book of Joshua (4:19), Joshua and the Israelites crossed the Jordan River at Gilgal with the Ark of the Covenant. It was the first great aliyah, calculated as being in the year 2488 (1272 BCE). God commanded the Israelites to mark the occasion by erecting twelve stones - for the twelve tribes - containing the text of the Torah.
10 Nisan also was the first Shabbat HaGadol, five days before the Israelites left Egypt at the beginning of the Exodus. It was also the date that Miriam, Moses’ sister, passed away.
Aliyah (עֲלִיָּה) means “a going up,” an ascending, in this case into the Land of Israel. A person who “makes Aliyah” is an “oleh” (masculine, plural “olim”) or an “olah” (feminine, plural “olot”).
So why did they choose 7 Heshvan for the school observance? Because we usually are reading Lekh-Lekha at that time, the parashah in which God orders Abraham to leave his home and family and go to the Land of Israel.