Which feast does Leviticus 23:5 say begins at twilight on the fourteenth day of the first month?

Answer: The Passover

Leviticus 23:5 sets the LORD's Passover at twilight on the 14th of the first month; the Feast of Unleavened Bread began the next day.

“The Passover to the LORD begins at twilight on the fourteenth day of the first month. On the fifteenth day of the same month begins the Feast of Unleavened Bread to the LORD. For seven days you must eat unleavened bread.”— Leviticus 23:5-6 (BSB)