Khag haShavuot  חַג הַשָׁבוּעוֹת
 
The Feast of Weeks - Shavuot is celebrated 50 days after the first Sunday of the paschal week. Shavuot must always fall on Sunday! On the day after Shabbat Mimakharat haShabbat מִמָּחֳרַת הַשַׁבָּת, which falls on paschal week, on Sunday, the first sacrificial sheaf of the new sacrificial harvested barley was brought. This sacrifice was called the sheaf of wave offering Omer hatenufa עׂמֶר הַתְּנוּפָה. It was the thanksgiving sacrifice to the Creator for the harvested crop, after this sacrifice we were allowed to eat bread prepared from the new crop.

From the day this  sacrificial sheaf was offered, the ancient Israelites counted seven weeks,  which is 49 days and on the 50th day they celebrated the Feast of Weeks - Shavuot חַג הַשָׁבוּעוֹת. On this day Israelites offered bread baked from fresh wheat flour Lekhem bikurim לֶחֶם בִּכּוּרִים as a sacrifice to God. This feast was the feast of the harvest and was also known as Khag hakatzir חַג הַקָּצִיר.

Nowadays it is customary to count individual days from the Sunday of the paschal week and it is customary to count them during the morning prayer to remind us  which of the 49 days of counting falls on each morning. This counting is called Sefirah סְפִירָה. On the Fiftieth day we celebrate the Feast of Weeks - Shavuot חַג הַשָׁבוּעוֹת Khag haShavuot, as it is written:

And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete; even unto the morrow after the seventh week shall ye number fifty days; and ye shall present a new meal-offering unto the יהוה. Ye shall bring out of your dwellings two wave-loaves of two tenth parts of an ephah; they shall be of fine flour, they shall be baked with leaven, for first-fruits unto the יהוה. (3M 23:15-17)
 
The feast of Shavuot is a day of worship (Holy) Assembly מִקְרָא קדֶשׁ Mikra Kodesh, in the same way as the first and last day of Passover. During the Feast of Shavuot all work is prohibited except the preparation of food for this feast. On these days special public worship takes place and specific prayers are recited which are called Tefilat Khag haShavuot תְּפִלַּת חַג הַשָׁבוּעוֹת.

And ye shall make proclamation on the selfsame day; there shall be a holy convocation unto you; ye shall do no manner of servile work; it is a statute for ever in all your dwellings throughout your generations. (3M 23:21)