This week’s Torah portion, Vayera (“and he appeared”), is packed full of some of the most exciting narratives in Genesis. There is the visit of the three angels to Abraham, the announcement of the birth of Isaac, the destruction of Sodom and Gomorrah, and last but not least, the Akeidah (the binding of Isaac). But hidden in these narratives is something that I feel is of great significance. There is evidence of a Passover-type celebration that pre-dates the Exodus and Mount Sinai.
In Genesis 18:10 Abraham is promised that Sarah will give birth to a son:
The LORD said, “I will surely return to you about this time next year, and Sarah your wife shall have a son.” And Sarah was listening at the tent door behind him. (Genesis 18:10)
Where the ESV has “next year, the Torah reads ka’et chayyah which literally translates as “at the time of life” which can be translated as “in the spring,” i.e., “I will surely return to you in the spring.” Spring is the time of the Passover festival. Rashi comments:
At this time, next year. It was Passover, and on the following Passover, Isaac was born since we do not read k’et [at “a” time] but ka’et [at “this” time]. 
Down in verse 10 we get another clue:
Is anything too hard for the LORD? At the appointed time I will return to you, about this time next year, and Sarah shall have a son. (Genesis 18:14)
“Appointed time” is the Hebrew word mo'ed which is the same word used for the “appointed times” of Leviticus 23. The Talmud comments:
It has been taught: R. Eliezer says: … on Passover Isaac was born. How do we know that Isaac was born on Passover?—Because it is written, “On the [next] festival (mo'ed) I will return unto thee.” (b.Rosh HaShanah 10a-11a)
Then later on in Sodom we see that Lot feeds the angels a feast of matzah:
But he pressed them strongly; so they turned aside to him and entered his house. And he made them a feast and baked unleavened bread, and they ate. (Genesis 19:3)
Matzah, of course, is one of the main components of the Passover festival. Lot would soon experience his own personal Exodus from Egypt.
Sarna notes that in Exodus 12:8 matzah is “introduced without definition and without explanation. The implication, justified by the biblical texts, is that matsah is already well known.”  The Midrash even states that the mitzvah of unleavened bread was given “in honor of Sarah who prepared cakes for the angels, though they did not taste bread” (Exodus Rabbah 14:2).
Based on this evidence, there is even a traditional piyut (liturgical hymn) that alludes to this tradition that is sung at the conclusion of the second Passover Seder and at shacharit prayers for the second day of Passover:
To the Oriental (Abraham) You revealed the future midnight of Passover.
At his door You knocked in the heat of the day on Passover.
He satiated the angels with matza-cakes on Passover.
And he ran to the herd—symbolic of the sacrificial beast of Passover.
The Sodomites provoked (God) and were destroyed by fire on Passover.
Lot was withdrawn from them, he had baked matzot at the time of Passover. 
From my perspective, it seems that if there was a Passover-like celebration that was observed before the instructions given at the Exodus from Egypt, it was akin to Chag HaMatzot, the week-long Festival of Unleavened Bread, and that the institution of Pesach (the actual Passover sacrifice) was added in Egypt.
Although we may never know whether this is mere speculation or actual fact, it should be pointed out that the festivals were first introduced in the creation account:
And God said, “Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons [mo'adim], and for days and years, and let them be lights in the expanse of the heavens to give light upon the earth.” And it was so. (Genesis 1:14-15)
“Seasons” is the Hebrew word mo'adim which, 100% of the time in the Torah, refers to the festivals.  For us as God-fearers this should be exciting. Although the festivals were later committed and entrusted to the Jewish nation, they contain universal truths and applications for all mankind to celebrate. Therefore, Parashat Vayera may well indeed give evidence of individuals celebrating Chag HaMatzot before it became directly associated with the Exodus from Egypt.
- Rabbi A. J. Rosenberg, Genesis: A New English Translation Mikraot Gedolot [New York, NY: The Judaica Press, 1993], 211b.
- Nahum Sarna, The JPS Torah Commentary: Genesis (Philadelphia, PA: Jewish Publication Society, 1989), 55.
- Translation from Rabbi Avie Gold, The Complete Artscroll Machzor Pesach: Sefard (Brooklyn, NY: Mesorah: 2001), 145.
- David J. Rudolph, “Festivals in Genesis 1:14,” Tyndale Bulletin 54, no. 2 (2003):23-40.