A Sunday Fruitfruits and Pentecost being counted from that date ending up on a Sunday is taught by Chuck Missler and J R Church because the Feasts of the Lord as defined by God are in Leviticus 23 proclaim it.
Leviticus 23:9 The Feast of Firstfruits (Always on a Sunday)
And the LORD spoke to Moses, saying, 10 "Speak to the children of Israel, and say to them:'When you come into the land which I give to you, and reap its harvest, then you shall bring a sheaf of the firstfruits of your harvest to the priest. 11 He shall wave the sheaf before the LORD, to be accepted on your behalf; on the day after the Sabbath (Reserection Sunday) the priest shall wave it.
Leviticus 23:15 The Feast of Weeks (Pentecost counted from Firstfruits Sunday)
And you shall count for yourselves from the day after the Sabbath, from the day that you brought the sheaf of the wave offering: seven Sabbaths shall be completed.