When is Pentecost ?
Shavuot, ('seven weeks') (pronounced: shah-voo-OH-t) is one of the three Biblical pilgrimage festivals.
It is a major Jewish holiday, and is also known as the Feast of Weeks. Greek-speaking Jews gave it the name Pentecost since it occurs fifty days after Passover.
If you don't count Passover, the holiday is 49 days after Passover, which is a jubilee of days. This ends the Counting of the Omer.
Shavuot has many aspects and as a consequence has been called by many names. In the Hebrew Bible it is called the "Feast of Harvest" (Hebrew: חג הקציר, Hag ha-Katsir; Ex. xxiii. 16) and the "Feast of Weeks" (Hebrew: חג שבעות, Hag Shavuot; ib. xxxiv. 22; Deut. xvi. 10), also the "Day of the First-Fruits" (Hebrew יום הבכורים, Yom ha-Bikkurim; Num. xxviii. 26).
Local customs include displaying greens on the floors and of otherwise decorating the home and the synagogue with plants, flowers, and even with trees.
The greens serve to remind one of the green mountain of Sinai; the trees, of the judgment day for fruit-trees on Pentecost; they also commemorate the harvest festival of former times.