Numbers 28:26-31. In the midst of his rules for his newly freed people, God tells them that on the day of first fruits, their feast of weeks, they shall have a holy convocation. On that day they they shall do no laborious work and they shall offer a cereal offering and an offering of two young bulls, one ram, and seven male lambs a year old, all of which shall be without blemish.

We can see the beginnings of the Mass here. God sets aside a weekly day of celebration and rest where the people gather and offer cereal offerings and sacrificial animal offerings that must be without blemish. At Mass we offer bread and wine that becomes the body and blood of the unblemished Lamb of God.  And just as God rested on the seventh day, saying "It is good," the Israelites are to rest on the seventh day and offer some of the best of what they have, of what is good, to the Lord. The sacrifice of these things takes place during the convocation, in the midst of all the people, just like the sacrifice of the Mass takes place with the congregation.

This really is a very very like the Mass. It's amazing how God prepared us all for this celebration from the very beginning!