Deuteronomy 16:10-11

10Then celebrate the Festival of Weeks to the LORD your God by giving a freewill offering in proportion to the blessings the LORD your God has given you. 11And rejoice before the LORD your God at the place he will choose as a dwelling for his Name—you, your sons and daughters, your male and female servants, the Levites in your towns, and the foreigners, the fatherless and the widows living among you.