Pentecost (Le 23)
15 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 be complete:
16 Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD.
17 Ye shall bring out of your habitations two wave loaves of two tenth deals: they shall be of fine flour; they shall be baken with leaven; they are the firstfruits unto the LORD.
18 And ye shall offer with the bread seven lambs without blemish of the first year, and one young bullock, and two rams: they shall be for a burnt offering unto the LORD, with their meat offering, and their drink offerings, even an offering made by fire, of sweet savour unto the LORD.
19 Then ye shall sacrifice one kid of the goats for a sin offering, and two lambs of the first year for a sacrifice of peace offerings.
20 And the priest shall wave them with the bread of the firstfruits for a wave offering before the LORD, with the two lambs: they shall be holy to the LORD for the priest.
21 And ye shall proclaim on the selfsame day, that it may be an holy convocation unto you: ye shall do no servile work therein: it shall be a statute for ever in all your dwellings throughout your generations.
22 And when ye reap the harvest of your land, thou shalt not make clean riddance of the corners of thy field when thou reapest, neither shalt thou gather any gleaning of thy harvest: thou shalt leave them unto the poor, and to the stranger: I am the LORD your God.
On the Feast of Firstfruits we are told to start counting fifty days, with Firstfruits being Day 1, to determine the date of another feast, the Feast of Pentecost. This feast appears to correspond to the giving of the Torah on Mount Sinai, which was about 50 days after the Exodus from Egypt, and is also the day on which the first large harvest of the Jewish people took place as three thousand souls were added to the kingdom when they received the preaching of Peter in Acts 2. (Ac 2:41) Celebrating this feast appears to be a celebration of the harvest, similar to Firstfruits.
While the above Levitical text describes the sacrificial duties of this festival, as with the other feasts Deuteronomy provides additional helpful practical details:
09 Seven weeks shalt thou number unto thee: begin to number the seven weeks from such time as thou beginnest to put the sickle to the corn.
10 And thou shalt keep the feast of weeks unto the LORD thy God with a tribute of a freewill offering of thine hand, which thou shalt give unto the LORD thy God, according as the LORD thy God hath blessed thee:
11 And thou shalt rejoice before the LORD thy God, thou, and thy son, and thy daughter, and thy manservant, and thy maidservant, and the Levite that is within thy gates, and the stranger, and the fatherless, and the widow, that are among you, in the place which the LORD thy God hath chosen to place his name there.
12 And thou shalt remember that thou wast a bondman in Egypt: and thou shalt observe and do these statutes. (De 16:9-12)
16 Three times in a year shall all thy males appear before the LORD thy God in the place which he shall choose; in the feast of unleavened bread, and in the feast of weeks, and in the feast of tabernacles: and they shall not appear before the LORD empty:
17 Every man shall give as he is able, according to the blessing of the LORD thy God which he hath given thee. (De 16:16-17)
Firstly, we may notice that the primary concept associated with this particular feast is that of a special “tribute of a free will offering,” (vs 10) which each man was required to bring and give to God. (De 16:16-17) A tribute is a gift intended to show gratitude, respect, or admiration and, as noted in Unleavened Bread, a freewill offering is evidently an animal sacrifice symbolic of voluntarily giving our entire selves to God. We can see here that it is particularly important for men, as leaders of our families and communities, to be examples of godliness for all to follow and to encourage each other in pursuing humble, sacrificial holiness in their family and cultural relationships.
By looking at both of these texts together we find God calling us in this feast to separate ourselves in community from our routines into a special place to do four things:
 rejoice with family, community and others interested in attending;
 remember our deliverance from slavery and bondage;
 renew our dedication and focus in obeying God’s commandments;
 voluntarily offer our lives to God, especially men, putting ourselves at His disposal to do with as He will.
As seen in the expression of this command in Deuteronomy, the formal command to actually count the days, one day at a time, to actually assign numbers to them and ceremonially count them each day as they occur, may not be the intent of the command as much as to determine the Sunday on which Pentecost falls. Certainly, counting the days one by one as they occur is not wrong, and may be a wonderful way to stay engaged with this waiting process between the two feasts, and will likely serve to heighten our engagement with and anticipation of this final harvest celebration, but it is unclear whether this type of counting is strictly commanded in the text.
The prophetic significance of Pentecost, also called Shavuot, seems related to three things:
 spring harvest (Ex 23:16);
 giving of Torah (which likely occurred about this time), and
 the outpouring of the Spirit to empower God’s harvest. (Ac 1:8)
All of these appear to be related to God’s work of sowing His Word into Man and reaping a harvest of righteousness as He transforms men by the power of the Spirit and the Word into His likeness. Perhaps this will be fulfilled in the ultimate going forth of the Word and the Spirit from Jerusalem to the nations during the millennium as God reaps His ultimate harvest in Man. (Is 2:2-3, Mic 4:2)
So, what do we do when Passover falls on a Sunday: should we begin to count the days to Pentecost from the Sunday of Passover when Passover Sunday?
The answer depends on which authority we follow, but I would say, based on Leviticus 23:15-6 (above), that we should wait a week before we start, and not count the Sunday of Passover as the first of the 50 days. We need to count 7 sabbaths after Passover, from the Sunday following Passover when the wave sheaf is offered, so there must be seven sabbaths in between Passover and Pentecost. So if Passover falls on a Saturday (starting Friday evening) then the next day is the first day after the Sabbath, and would count toward Pentecost. However, if Passover falls on a Sunday (starting Saturday evening) then wait a full week until the next Sunday, after a weekly sabbath has passed, to start counting toward Pentecost.