Fast of the firstborn[1]
הבכורות מתענין בערב פסח בין בכור מאב בין בכור מאם ויש מי שאומר שאפילו נקבה בכורה מתענה: (ואין המנהג כן)
The firstborns fast on the day before Pesach, whether they are the firstborn of their father or firstborn of their mother. Some say even firstborn women fast (Gloss: but this isn’t the custom)[2]
There’s an ancient custom[3] for the firstborn to fast on Erev Pesach, the day before Pesach. The common explanation[4] for this fast day is that it’s in commemoration of The Plague of the First Born. The last of the Ten Plagues, all the firstborn Egyptians died at midnight. All the firstborn of the Jews were miraculously saved, so every year right before Pesach the firstborn fast. This sounds like it’s due to the gratitude of the firstborns that they fast.