The holy days are connected to significant historic happenings, such as the Exodus from Egypt on Passover, the receiving of the Torah on Mount Sinai on Shavuot (Pentecost), or the wanderings of the Children of Israel in the wilderness on Succot (Tabernacles).
These holy festivals are not intended simply as memorial days to keep alive the memory of the events.
They are divinely appointed times dedicated to a renewal of the same revelation that once occurred on that day in the year, a repetition and a restoration of the same forces.
So that the sanctity of the holidays is derived not only from a primal divine revelation but also from Israel's continual resanctification, in the way it keeps these days holy, of this revelation.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose, p. 77, by Rabbi Adin Steinsaltz