Pentecost

Ex. 34:22 And thou shalt observe the feast of weeks, even of the first-fruits of wheat harvest, and the feast of ingathering at the year’s end.

Num. 28:26 Also in the day of the first-fruits, when ye offer a new meal-offering unto the Lrd in your feast of weeks, ye shall have a holy convocation; ye shall do no servile work.

Deut. 16:9-12 Seven weeks shalt thou number unto thee: from the time thou beginnest to put the sickle to the standing grain shalt thou begin to number seven weeks. And thou shalt keep the feast of weeks unto the Lrd thy Gd with a tribute of a freewill-offering of thy hand, which thou shalt give, according as the Lrd thy Gd blesseth thee: and thou shalt rejoice before the Lrd thy Gd, thou, and thy son, and thy daughter, and thy man-servant, and thy maid-servant, and the Levite that is within thy gates, and the sojourner, and the fatherless, and the widow, that are in the midst of thee, in the place which the Lrd thy Gd shall choose, to cause his name to dwell there. And thou shalt remember that thou wast a bondman in Egypt: and thou shalt observe and do these statutes.

Acts 2:1-4 And when the day of Pentecost was now come, they were all together in one place. And suddenly there came from heaven a sound as of the rushing of a mighty wind, and it filled all the house where they were sitting. And there appeared unto them tongues parting asunder, like as of fire; and it sat upon each one of them. And they were all filled with the Holy Spirit, and began to speak with other tongues, as the Spirit gave them utterance.

First Day of Feast of Unleavened Bread

The first day of the Feast of Unleavened Bread is a sabbath day of rest.

Lev. 23:4-8 These are the set feasts of the Lrd, even holy convocations, which ye shall proclaim in their appointed season. In the first month, on the 14th day of the month at even, is the Lrd’s passover. And on the 15th day of the same month is the feast of unleavened bread unto the Lrd. Seven days ye shall eat unleavened bread. In the first day ye shall have a holy convocation: ye shall do no servile work. But ye shall offer an offering made by fire unto the Lrd 7 days. In the 7th day is a holy convocation; ye shall do no servile work.

Day of First-Fruits

The Hebrew Bible says the priest shall wave the sheaf on the day after the sabbath. No bread, parched grain, or fresh ears were allowed to be eaten until this day.

Lev. 23:9-14 (Hebrew) And the Lrd spake unto Moses, saying, Speak unto the children of Israel, and say unto them, When ye are come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring the sheaf of the first-fruits of your harvest unto the priest: and he shall wave the sheaf before the Lrd, to be accepted for you: on the morrow after the sabbath the priest shall wave it. And in the day when ye wave the sheaf, ye shall offer a he-lamb without blemish a year old for a burnt-offering unto the Lrd. And the meal-offering thereof shall be two tenth parts of an ephah of fine flour mingled with oil, an offering made by fire unto the Lrd for a sweet savor; and the drink-offering thereof shall be of wine, the fourth part of a hin. And ye shall eat neither bread, nor parched grain, nor fresh ears, until this selfsame day, until ye have brought the oblation of your Gd: it is a statute for ever throughout your generations in all your dwellings.

The Greek Bible says the priest shall wave the sheaf on the day after the first day of the Feast of Unleavened bread, which is a sabbath day..

Lev. 23:11 (Greek) And he shall raise up the sheaf before the Lrd, acceptable for you; on the day after the first the priest shall raise it up.

Day of Pentecost

The Hebrew and Greek Bible say to count 50 days from the day after the sabbath.

Lev. 23:15-21 (Hebrew) And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave-offering; seven sabbaths shall there be complete, even unto the morrow after the seventh sabbath shall ye number 50 days; and ye shall offer a new meal-offering unto the Lrd. Ye shall bring out of your habitations two wave-loaves of two tenth parts of an ephah: they shall be of fine flour, they shall be baken with leaven, for first-fruits unto the Lrd. And ye shall present with the bread seven lambs without blemish a year old, and one young bullock, and two rams: they shall be a burnt-offering unto the Lrd, with their meal-offering, and their drink-offerings, even an offering made by fire, of a sweet savor unto the Lrd. And ye shall offer one he-goat for a sin-offering, and two he-lambs a year old for a sacrifice of peace-offerings. And the priest shall wave them with the bread of the first-fruits for a wave-offering before the Lrd, with the two lambs: they shall be holy to the Lrd for the priest. And ye shall make proclamation on the selfsame day; there shall be a holy convocation unto you; ye shall do no servile work: it is a statute for ever in all your dwellings throughout your generations.

Lev. 23:15-16 (Greek) And from the day after the sabbaths, from the day on which you bring the sheaf of the addition, you shall count off seven whole weeks. You shall count until the day after the last week, 50 days, and you shall present a new sacrifice to the Lrd.

Josephus

Josephus says the day of first-fruits is the 16th day of the month, the day after the first day of the Feast of Unleavened Bread .

“The feast of unleavened bread succeeds that of the passover, and falls on the 15th day of the month, and continues 7 days, wherein they feed on unleavened bread; on every one of which days two bulls are killed, and one ram, and seven lambs. Now these lambs are entirely burnt, besides the kid of the goats which is added to all the rest, for sins; for it is intended as a feast for the priest on every one of those days. But on the 2nd day of unleavened bread, which is the 16th day of the month, they first partake of the fruits of the earth, for before that day they do not touch them. And while they suppose it proper to honor Gd, from whom they obtain this plentiful provision, in the first place, they offer the first fruits of their barley, and that in the manner following: They take a handful of the ears, and dry them, then beat them small, and purge the barley from the bran; they then bring one tenth deal to the altar, to Gd; and, casting one handful of it upon the fire, they leave the rest for the use of the priest; and after this it is that they may publicly or privately reap their harvest. They also at this participation of the first-fruits of the earth, sacrifice a lamb, as a burnt offering to Gd. When a week of weeks has passed over after this sacrifice (which weeks contain 49 days), on the 50th day, which is Pentecost, but is called by the Hebrews Asartha, which signifies Pentecost, they bring to Gd a loaf, made of wheat flour, of two tenth deals, with leaven; and for sacrifices they bring two lambs.
——Josephus, Antiquities of the Jews, Book 3, Chapter 10, Section 5-6, Written 93 AD.

Philo

“There is also a festival within the [Passover] festival which is immediately after the first day, and this is named the sheaf, from what takes place on it; for the sheaf is brought to the altar as a first-fruit both of the country which the nation has received for its own, and also of the whole land; so as to be an offering both for the nation separately, and also a common one for the whole race of mankind.”
——Philo, On the Special Laws, Book 2, Section 162, Written c. 20-50 AD.