[Avodah] Fast of the First Born.

[Martin Brody via Avodah]

Akiva Miller posted
"My eldest grand-daughter asked me why girls don't go to the siyum on
Erev Pesach, given the medrash firstborn girls *were* killed in Makas
Bechoros etc."

Maybe because the fast has nothing to do with the 10th plague at all?
They should be celebrating, not fasting! Fasting is for repentance.
More likely the Sin of the Golden Calf, when the firstborn lost their
priestly status. On the 14th the first born have to watch the Aaronite
priests slaughtering the Pascal offering instead of them.
Cheers
Martin Brody