Matthew chapter 27:52 says that the graves were opened and the saints were raised. My question is: Who were those saints?
They were Old Testament believers in a coming Messiah. While they didn’t live to see Him come, they believed that He would and when He did, He would save them from their sins. Their faith was credited to them as righteousness and when Jesus came out of the grave he brought them along with Him.
They are sometimes referred to the fulfillment of the wave offering, or sample of the harvest, that was part of the Feast of First Fruits ceremony (Lev. 23:9-11).