Richard Agee explains that the Day of Blowing Trumpets, commonly known as Rosh Hashanah, is the start of the cycles of seven found throughout the Bible referring to “rest” (the Hebrew words for both “rest” and “seven” are similar): seventh day of the week is the day of rest, the Shabbat (Gen. 2:3; Ex. 20:8-10); every seventh year the land must rest (Ex. 23:11); seventh group of seven years, or the Jubilee (Lev. 25).
Richard considers another possibility other than the three main methods for determining the timing of Shavuot/Pentecost related to encampments the people of Israel during their exodus from slavery in Egypt leading up to their receiving the 10 Commandments from God at Mt. Sinai. Richard contends that a connection to the weekly Shabbat has an important bearing on what God is trying to teach with the festivals of Passover, Unleavened Bread, Firstfruits and Shavuot.