Most folks who celebrated Pentecost did it on the Sunday 20th May. The same folks celebrated Passover on a Saturday 7th April, 7 weeks prior. Most folks believe the count starts "on the morrow AFTER the (annual) Sabbath". But the greek and Hebrew state that the count starts on the Sabbath morrow/morning, not the morrow or morning AFTER the Sabbath.
For most who don't celebrate Pentecost, this all sounds very technical.
Lev 23:11
And he shall wave the sheaf before the LORD, to be accepted for you: on the morrowafterthe sabbath the priest shall wave it.
Lev 23:15
And ye shall count unto you from the morrowafterthe sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete:
VAYIKRA/LEVITICUS 23:11
He shall wave the Omer before HASHEM to gain favor for you; on the morrow of the rest day the Kohen shall wave it.
VAYIKRA/LEVITICUS 23:15
You shall count for yourselves -- from the morrow of the rest day, from the day when you bring the Omer of the waving -- seven weeks, they shall be complete.