Deuteronomy 12:18

But thou must eat them before the LORD thy God in the place which the LORD thy God shall choose, thou, and thy son, and thy daughter, and thy manservant, and thy maidservant, and the Levite that is within thy gates: and thou shalt rejoice before the LORD thy God in all that thou puttest thine hands unto.

Thou must eat them ... thy man-servant, and thy maid-servant ... Servants, even foreign slaves, had many rights and privileges among the Israelites. One of these was that they participated in the public rejoicings on the great festivals, and were specially invited to certain feasts (cf. Deuteronomy 16:11).