The festival Sabbaths are NOT the weekly Sabbath.
“The Jewish calendar is lunisolar; that is, the months are reckoned according to the appearance of the moon, and the year of twelve or thirteen months, according to the sun. To balance the lunar months which have either 29 or 30 days each, totaling 354 days in a purely lunar year, with the solar year of 365 or 366 days, in seven out of the 19 years of the lunar cycle, an extra month --Adar II is added.” The Mount Zion Reporter
THE WEEKLY SABBATH begins at sunset of the sixth day.
“And God made two great lights; the greater light to rule the day, …” Gen 1:16
Gen_2:2 And on the SEVENTH DAY God ended his work which he had made; and he rested on the seventh day from all his work which he had made.
Gen_2:3 And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.
Since God sanctified the seventh-day Sabbath at creation, it has always been the seventh day of the week; from sundown of the sixth day to sundown of the seventh day. It has never been changed. During the seasons when the festival Sabbaths were observed, the weekly Sabbath was kept at its regular time and was called an “Intermediate Sabbath” by the Jews.
THE FESTIVAL SABBATHS are calculated from the appearance of the New Moon.
Passover was counted from the new moon of Nisan/Abib.*
NOTE: THE WORDS IN BRACKETS ARE MINE.
Lev_23:5 In the fourteenth day of the first month at even is the LORD'S passover. [Nisan 14]
Deu 16:1 Observe the month of Abib [aka as Nisan], and keep the passover unto the LORD thy God: for in the month of Abib the LORD thy God brought thee forth out of Egypt by night.
Deu 16:2 Thou shalt therefore sacrifice the Passover [Nisan 14] unto the LORD thy God, of the flock and the herd, in the place which the LORD shall choose to place his name there.
Deu 16:3 Thou shalt eat no leavened bread with it; [Nisan 15] seven days shalt thou eat unleavened bread therewith, even the bread of affliction; for thou camest forth out of the land of Egypt in haste: that thou mayest remember the day when thou camest forth out of the land of Egypt all the days of thy life.
Deu 16:4 And there shall be no leavened bread seen with thee in all thy coast SEVEN DAYS; [to Nisan 22] neither shall there any thing of the flesh, which thou sacrificedst the first day at even, remain all night until the morning.
Deu 16:5 Thou mayest not sacrifice the passover within any of thy gates, which the LORD thy God giveth thee:
Deu 16:6 But at the place which the LORD thy God shall choose to place his name in, there thou shalt sacrifice the passover at even, at the going down of the sun, at the season that thou camest forth out of Egypt.
Deu 16:7 And thou shalt roast and eat it in the place which the LORD thy God shall choose: and thou shalt turn in the morning, and go unto thy tents.
Deu 16:8 Six days thou shalt eat unleavened bread: and on the seventh day [Nisan 22] shall be a solemn assembly to the LORD thy God: thou shalt do no work therein. [“No work, indicates a Sabbath; the second sabbath of seven festival weeks.]
Deu 16:9 ***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. [Nisan 16 which is the morrow after the festival Sabbath of Nisan 15]
Deu 16:10 And thou shalt keep the feast of weeks [Pentecost] 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:
Summary of the count to Pentecost: From Nisan 16 to Pentecost is seven weeks. 7 x 7 =49. Nisan 16 is the morrow after the Sabbath of unleavened bread. 49 +1 = 50.
* Nisan/Abid: Abib is the Canaanite name of the first month, called Nisan in the Akkadian language of the Babylonians.