Well, Firstfruits begins the day after the first Sabbath Day following the Day of Passover (the 14th).I know there are High Holy Days and one of those is the day after Passover, I think.
Passover (day lamb is killed) is always on the 14th day of the month, no matter what day of the week it falls on.
Unleavened Bread always starts on the 15th.
DATE = 1st, 15th, 22nd, etc.
DAY = Monday, Wednesday, Friday. etc.)
But firstfruits is never given a date (14th, 15th, etc.) because it can only happen on Sunday (the day after the Sabbath Day), therefore, the date will change every year of Firstfuits, but the day is always the same - Sunday.
Whereas the dates of Passover & Unleavened Bread never change, but the days they fall on do change.
Passover & Unleavened Bread --- day changes, but the date is always the same.
Firstfruits ------------------------- date changes, but the day is always the same.