1 - Nisan 15 (Abib 15) - Passover. The first day of the seven-day-long Feast of Unleavened Bread. Always very early in Spring, in the 1st month.
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2 - Shavu`ot (Pentecost) - 50 days from the Sunday that follows Passover. A Sunday which is 50 days from the Sunday within the 7-day-long Feast of Unleavened Bread. Always very late in Spring, in the 3rd month. In the Bible, this feast is also known as The Feast of ~Weeks~ and The Feast of ~Harvest~.
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3 - Tishri 15 (Sukk`ot) - The first day of the seven-day-long Feast of Tabernacles (Booths). 15 days from Rosh haShanah and 5 days from Yom Kippur. Can (rarely) be very late in Summer, but usually is very early in Fall, in the 7th month. In the Bible, this feast is also known as The Feast of ~Ingathering~. 
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