The 7th day Sabbaths in the Feast of Passover & Unleavened Bread.

Scripture uses numbers interchangeably for both the feast day number and also for the day number of the weekly cycle, for example, the first day take branches and build a booth means the first day of the weekly cycle, not the first day of the Feast of Tabernacles which is the 7th day Sabbath.

Exodus 12:18, 19
18 In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even. (7 night portions of days, and 7 daylight portions of days) A day unit number consists of: sunrise, begins the number of a new day, and the night portion following it still has the same number assignement, as illustrated throughout scripture.

19 Seven days shall there be no leaven found in your houses:     (for 7 daylight portions of 7 days, there was no leaven in the house, 15th-21st, the night portion of the 14th day, is not called a "day", only the "daylight" portion is called a "day") Day, or Yom, can be the daylight, or day-unit of time. 14th day, locates the 24 hour block, "night" tells when to begin "no leaven found in house", it is disposed of in the 14th daylight.

Exodus 34:18
18 ¶ The feast of unleavened bread shalt thou keep. Seven days thou shalt eat unleavened bread, as I commanded thee, in the time of the month Abib: for in the month Abib thou camest out from Egypt.

Exo 20:9 Six days shalt thou labour, and do all thy work:
Exo 20:10 And day the seventh is shabat, unto Yehovah thy Elohim: in it thou shalt not do any work,

Deuteronomy 16:8
8 Six days (the six work days of the weekly cycle are the last 6 days of eating unleavened bread ending on the 21st at even) thou shalt eat unleavened bread: and on the seventh day(of the weekly cycle which the feasts are synchronized with) [shall be] a solemn assembly to YHVH thy Elohim: thou shalt do no work [therein].

Exodus 12:18
18 In the first [month], on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even.

Exodus 13:6
[Seven]-7651   [days]-3117    [you shall eat]-398   [unleavened bread]-4682,   [and on day]-3117   [ha=the seventh]-7637   [a feast]-2282(solemn assembly-sacrifice)   [to Y'hovah]-3068.

Exodus 12:16
And on day the first {first day of feast, the 7th day sabbath, the 15th day of the moon} a convocation holy and in day the seventh {7th day of week} a convocation holy shall be to you any work not shall be done on them; {the beginning and ending Sabbaths of this feast} only what must be eaten by each soul, that alone may be done by you.

Our Heavenly Father does not contradict Himself, we already know He commanded 6 working days, and here he does not contradict Himself, the 7th day commanded to keep from work is not one of the 6 working days He commanded "Thou shalt labor", but is the 7th day of the weekly cycle.

This feast of unleavened bread is an eight day occaision like tabernacles is, but on the eighth day you can eat leavened bread, and is called a chag, and sabbath. The previous day, the 6th day of the weekly cycle, is not a sabbath day, but is the 7th day of this feast, the 6th day of the week, the preparation day, and last day of fasting from leaven, which started on a 7th day sabbath.  Unleavened bread only is referred to as lasting 7 days.

Only by the lunar calendar weeks can all these texts be understood, and without conflicts, for if the text above referred to the 7th day of the feast which would be the 6th day of the week, the 21st day of the moon, then there would be two sabbaths in a row, if the 7th day of Deut. 16:8 were the 21st. So it is obvious that the 6th day (of the weekly cycle) is the last day of unleavened bread which is the 21st(see Exodus 12:18 quoted above), and of course the 22nd day is the 7th day Sabbath.

Our Great Creator is The Master of all Perfections, Infinite Perfection. He made not the Sabbath to be a burden, you can prepare food to eat on the Sabbath for the Sabbath day, it is not a burden as some would make it to be, as shown by Exodus 12:16, the first and last days of the feast gathering, are both 7th day weekly Sabbaths, as the weekly cycles are stuck fast days tied to the lunar month. (Exodus 20:8 "Mark even day, the New Moon, to consecrate It.")

Numbers 9:2
2 Let the children of Israel also keep the passover at his appointed season.

Numbers 9:3
3 In the fourteenth day of this month, at even, ye shall keep it in his appointed season: according to all the rites of it, and according to all the ceremonies thereof, shall ye keep it.

In Exodus 12:16 and other texts, mentioning the first day of the feast and the 7th day, it means the 7th day of the week and not the 7th day of the feast.

Exodus 12:16
And in the first day (15th a 7th day sabbath) there shall be an holy convocation, and in the seventh day (Deuteromony 16:8 & Exodus 12:18 defines this seventh day as the 22nd, the 7th day sabbath, the chag-feast with leavened bread) there shall be an holy convocation to you: no manner of work shall be done in them, save that which every man must eat, that only may be done of you.

Exodus 12:16
And in the first day (of the feast of unleavened bread-the 15th is the first daylight of this feast, which began at even of the 14th day-yom-the darkness portion-which is still called the 14th until break of daylight) there shall be an holy convocation (this is the 7th day sabbath the 15th as Exodus 16 shows clearly, by counting backwards from the first Shabat upon which no manna fell) to you: no manner of work shall be done in them, ("them" plural means the 15th, a 7th day sabbath, and the 7th day of the following week, the 22nd, the "mimouna" feast with leavened bread, a part of the total span of 8 daylights [14th at even-night,  through the 22nd until evening--portions of 9 units of time, called "yom", but only 8 daylights, the daylight portion is also called a "yom"], as explained by Deuteronomy 16:8) save that which every man must eat, that only may be done of you.

Deuteronomy 16:8
Six days (the six working days of the weekly cycle) thou shalt eat unleavened bread: and on the seventh day (Shabat) shall be a solemn assembly to Y'hovah thy Elohim: thou shalt do no work therein. (uleavened bread is eaten for 7 daylight portions, and for 7 nightime portions of 8 consecutive time units-days, see chart above, and the 7th day, spoken of in this verse is the 7th day sabbath, the 22nd, in which you can feast upon leavened bread prohibited from the 14th at even until the 21st at even. see Exodus 12:18