Philo, who lived during the lifetime of the Messiah, confirms both Torah and the wording of the Septuagint.
He writes
For it is said in the Scripture: On the tenth day of this month let each of them take a sheep according to his house; in order that from the tenth, there may be consecrated to the tenth, that is to G-d, the sacrifices which have been preserved in the soul, which is illuminated in two portions out of the three, until it is entirely changed in every part, and becomes a heavenly brilliancy like a full moon, at the height of its increase at the end of the second Week...
S.: Philo ON MATING WITH THE PRELIMINARY STUDIES XIX. (102)
Philo confirms the statement of Ps.81, namely that the Passover at the end of the second week coincides with the full moon. The end of the second week, as seen from the new moon, falls around the 15th day
But to the seventh day of the week he has assigned [the beginning of] the greatest festivals, those of the longest duration [Unleavened Bread and Tabernacles], at the periods of the equinox both vernal and autumnal in each year;...so that each month might receive an especial honour of one sacred day of festival, for the purpose of refreshing and cheering the mind with its holiday.
S.: Philo THE DECALOGUE XXX (161)
This seventh is the weekly seventh and is in front of the six days during the feasts because to the weekly seventh day He has assigned these feasts.
In other words, the greatest (longest) feasts (the Feast of Unleavened Bread and the Feast of Tabernacles) were assigned to the seventh day of the lunar week (the 15th day of the month), with which the feast begins each time and lasts seven days. They were keeping lunar weeks because there is no way the weekly Sabbath (15th) can begin these two festivals on the 15th of 1st and the 15th of 7th month each year, on a continuous seven day cycle by the calendar of today.
Notice Philo DID NOT say they would receive two holy days of festivals, but one, the 15th. To prove the seventh day of the week is the same as the 15th, Philo states,
THE SPECIAL LAWS, II
XXXIII. (210) Again, the beginning of this festival is appointed for the fifteenth day of the month (or seventh day of the week), on account of the reason which has already been mentioned respecting the spring season,
Further proof of this found in John which states
“(for that sabbath day was an high day,)” John 19:31
The Pulpit Commentary notes
Consequently, there was a twofold sanctity about that particular sabbath, seeing that the sabbatic rest of the day following the Paschal meal coincided with the ordinary weekly sabbath; (for great, or high, was the day of that sabbath) (cf. Exodus 12:16; Leviticus 23:7; and notes on John 13:1; 18:28; 19:14). The Pulpit Commentary drew from over 100 authors over a 30 year span to assemble this conservative and trustworthy homiletical commentary set.
So here, just like Philo says, the first day of the Feast (whether of Unleavened Bread or Tabernacles) ALWAYS falls on the weekly Sabbath at the end of the second week, that is the 15th of the lunar month.
Elsewhere, he explains
The fourth (commandment), which treats of the Seventh Day, must be regarded as nothing less than a gathering under one Head of the feasts and the purifications ordained for each feast,
S.: Philo THE DECALOGUE XXX (159)
How can the weekly Sabbath day be considered an assembly under a head of the feasts if it is not at the head of those feasts, that means, begins each year with it?
This proves lunar Sabbaths.
Philo continues by saying,
THE DECALOGUE
XXX (159) “By the seventh I mean BOTH the seventh which INCLUDES the most creative of numbers, six, and that which does NOT INCLUDE it but takes precedence of it and RESEMBLES the unit.”
The word precedence means it comes BEFORE the number six during the feasts, i.e. one of the sevenths comes before the number six during the 7 day feast and the other seventh comes after it and is combined with it. This is IMPOSSIBLE if he used the count for the Sabbath as the people of today do. This seventh is the weekly seventh and is in front of the six days during the feasts because to the weekly seventh day He has assigned these feasts.
Last but not least it says,
“BOTH these are EMPLOYED by Him [YEHOVAH G-d] in reckoning the feast-times.”
You cannot reckon feast-times with a seventh that jumps around during the 7 day feast, on a man-made calendar. Both the sevens have to be fixed — not just the one that is on the 21st because He employed both sevens in reckoning the feast-times. If one of the sevenths could move it would also fall on the 21st at times and would also be combined with the number six and there would be only one seventh.
When Philo states that there are two sevenths in both the Festival of Unleavened Bread and the Festival of Booths (Tabernacles) is he somehow missing the point Saturday sabbatarians would bring up today — that there is a third seventh that will hit in between the 15th and the 21st the majority of years that the feasts come around?
Why does Philo not mention this third seventh?
It is because none exists. Philo only mention two sevenths in relation to the feast and the first of these two sevenths is none other than the weekly seventh day Sabbath that leads the feasts and is considered the first day of the feast — the 15th.
Creation follows the daily, weekly, and monthly rhythms, a rhythm that is struck in the very first verse of the Bible.
It contains in Hebrew exactly seven words corresponding to the seven-day week or a lunar phase, and counts exactly twenty-eight letters corresponding to the four-week sidereal month.
The new moon day is followed by four weeks consisting of six working days and a Sabbath. This results in the Sabbaths always falling on the 8th,15th,22nd and 29th of the month.