Now a days, Passover is usually celebrated on Thursday night. But, originally, it was celebrated on the first full moon of the month. And, this full moon could occur on different days of the week from one year to the next.

However, when Passover occurred on Thursday, not only did you have Thursday night and all day Friday as a holiday, you also had the Sabbath which went from Friday night to Saturday night. Thus a two day weekend.

I speculate that some time in the distant past, they decided that Thursday was the preferred day of the week for Passover so that everyone could have a long weekend.