See the notes at Matthew 24:42-51.
Second watch - See the notes at Matthew 14:25.

If he shall come in the second watch - See the note on Matthew 14:25.

And if he shall come in the second watch,.... Of the night, that is, after nine o'clock, or any time between nine or twelve; for the second watch was from nine o'clock till twelve; and this was coming early from an entertainment, or a wedding, which were commonly kept in the night, and late;
or come in the third watch, or after twelve o'clock, or any time between twelve and three; for the third watch was from twelve o'clock to three, which was late; See Gill on Matthew 14:25 The Persic version reads, "in the second, or third part of the night"; and the Ethiopic version, "in the second or third hour of the night";
and find them so. The Arabic version adds, "doing"; as above described, with their loins girt, lights burning, and they watching for their Lord's coming:
blessed are those servants; since they shall be used and treated as before related.

second . . . third watch--To find them ready to receive Him at any hour of day or night, when one might least of all expect Him, is peculiarly blessed. A servant may be truly faithful, even though taken so far unawares that he has not everything in such order and readiness for his master's return as he thinks is due to him, and both could and would have had if he had had notice of the time of his coming, and so may not be willing to open to him "immediately," but fly to preparation, and let his master knock again ere he admit him, and even then not with full joy. A too common case this with Christians. But if the servant have himself and all under his charge in such a state that at any hour when his master knocks, he can open to him "immediately," and hail his "return"--that is the most enviable, "blessed" servant of all.

The Jews frequently divided the night into three watches, to which our Lord seems here to allude.