Here's the Rashi comment in question :
ויפצר בם מאד ויסרו אליו ויבאו אל ביתו ויעש להם משתה ומצות אפה ויאכלו׃
But he insisted so strongly that they came with him and went into his home. He prepared a feast for them, and baked some flatbreads, and they ate.
Rashi: And baked some flatbreads: It was Passover
Asks Divrei Chaim:
What do we make of Rashi? Rashi seems to be addressing a simple textual question... If you dismiss Rashi as “derash” and not pshat, what does that mean? – did Rashi waste his time composing “fictional” answers to explain troublesome details in the text? Or to put it another way, if Rashi knew it was irrational or improbable for Lot to have really kept Pesach and eaten matzah, then hasn’t Rashi failed to answer the textual question he posed?"The problem, I think, is that DC has a post-enlightenment way of looking at history, and can't quite wrap his head around the fact that Rashi -for all his brilliance- did not. We're modern. Anachronisms bother us. Rashi was from a different time and place, a foreign country in LP Barthartley's famous phrase. Anachronisms didn't bother him (His commentary, and the Midrash upon which most of it is based, are chock full of them.) Nor did they bother the non-Jewish geniuses of his time (See Anselm of Canterbury who imagined God as the fuedal lord of the word, and interpreted the Jesus story from that perspective.) We worry about the little details of history. We want to know exactly what Lot served the visiting angels. Rashi was different. As a man of the 13th century, he just didn't care. He was a critic, concerned about the words on the page , and a halachist, concerned about Jewish law. He was not a historian in any sense, but certainly not a historian in the modern sense.
1 - I could argue that Rashi's view of 19:3 isn't necessarily anachronistic. Suppose we were to corner Rashi and ask him what happened in Sodom on the night before the destruction. We could ask him three different questions: (1) Were matzot really served?; (2) Was it really Pesach?; and (3) Did Lot bake matzot because he somehow knew that in the far off future his uncle's descendants would be commanded to eat them in memory of an event that was yet to occur? I'm certain Rashi would say "yes" to the first two questions, but if he were to answer "no" to the third, his comment isn't an anachronism.
2- In particular, Rashi wants to know why the word "feast" is used to describe a meal that consisted of the poor man's bread. His answer, despite what you may have heard about Rashi, didn't arrive via ruach haKodesh. Its right from Berashis Rabba. (Not having seen this particular midrash in many years, I can't tell you what its author was attempting to address, but I'd bet serious money that it was not written to tell us the facts of history. Midrashim rarely attempt to tell us what happened. They almost always serve to give us some lesson or explanation. Because Rashi often re purposes midrashim, the fact that Rashi uses this one to explain why the feast Lot prepared was so pathetic is not proof that the midrash was written for that reason, too.)
3 - This, incidentally, is the other problem with the DC approach. Along with construing Rashi as a historian, he construes midrash as history. True, the midrash says that the angles visited Lot on Passover. However, it doesn't follow from this that it actually was Passover or even that the author of the Midrash believed it himself. The Ramchal says that moral and metaphysical lessons are hidden in midrashim. Perhaps the author of this particular midrash said that the visit occured on Passover for the sake of some moral lesson he wished to convey.
4 - Or perhaps he really thought it was Passover. We have no way of knowing.
[*] Its also possible that the words ומצות אפה were added by a later scribe/editor/redactor who wanted to make an editorial point about the salvation of Lot and connect it to the later salvation of the Jewish people. This is the sort of thing Midrash does all the time, so it doesn't seem far fetched that an scribe/editor/redactor might try it, too. But that's another post