The eight day is considered to be a holy convocation or assembly. Such a gathering normally occurs as the end of a religious festival (for example, Deuteronomy 16:8). The Hebrews were to restrain themselves for one more day before resuming the activities of normal life.
36 For seven days you shall present food offerings to the LORD. On the eighth day you shall hold a holy convocation and present a food offering to the LORD. It is a solemn assembly; you shall not do any ordinary work.