Albert Barnes on Matthew 26:34

This night. This was in the evening when this was spoken, after the observance of the passover, and we may suppose near nine o'clock.

Before the cock crow. Mark and Luke add, before the cock crow twice. The cock is accustomed to crow twice, once at midnight, and once in the morning, at break of day. The latter was commonly called cock-crowing. See Mk 13:35. This was the time familiarly known as the cock-crowing; and of this Matthew and John speak, without referring to the other. Mark and Luke speak of the second crowing, and mean the same time; so that there is no contradiction between them.

Deny me thrice. That is, as Luke adds, deny that thou knowest me. See Mt 26:74.