Firstfruits falls during the 8-day celebration of Passover/Unleavened Bread. According to Leviticus 23:15, it falls on the day after the regular Sabbath during the festival. So this Feast always falls on a Sunday, whereas the first day of Passover falls on 14th day of Nisan according to the Hebrew calendar, which could be any day of the week.
The feast, as given in Leviticus 23, is to be a wave offering from the barley harvest. This barley harvest is the very first harvest in the agricultural year of Israel. Yeshua (Jesus) represents the firstfruits from the dead.