Albert Barnes Commentary Mark 13:35

"Watch therefore: for ye know not when the lord of the house cometh, whether at even, or at midnight, or at cockcrowing, or in the morning;" — Mark 13:35 (ASV)

Watch ye. Be diligent, faithful, and waiting for the return of your Lord, who will come at an unexpected hour.

Master of the house. Denoting here the Lord Jesus.

At even, or at midnight, or at etc.: This refers to the four divisions into which the Jews divided the night.