During the time when the YHWH's Temple stood in Jerusalem, this was a day when the two loaves of leavened wheat bread were presented to Him as a Wave Offering by the Priest. This was a first fruits offering for the wheat harvest, the day after this offering, the wheat harvest would commence. The day goes back much further than the temple though, Shavuot is the day that God gave the Ten Commandments to Moses on Sinai, 50 days after the exodus from Egypt.
For our family, the giving of Yahweh's instruction is a red letter day. It reminds us of how much He loves us, through His salvation, liberating us from the bondage of the oppressor, His instruction, the 10 commandments and their explanation to guide our way, and His provision for our need for food and shelter. We are so grateful and there is much cause to celebrate!