Albert Barnes Commentary Matthew 27:64

"Command therefore that the sepulchre be made sure until the third day, lest haply his disciples come and steal him away, and say unto the people, He is risen from the dead: and the last error will be worse than the first." — Matthew 27:64 (ASV)

Until the third day. That is, during two nights and the intervening day. This proves that when the Jews spoke of three days, they did not of necessity mean three whole days, but parts of three days, as was the case in our Saviour's lying in the grave.

The last error shall be worse than the first. That is, the last deception, or taking him from the tomb, pretending that he rose, shall have a wider influence among the people than the first, or his pretending to be the Messiah.