Exegesis:

For grēgoreō ‘watch’ cf. the preceding verse; ho kurios ‘the master,’ ‘the owner’ cf. 1.3.

Listed here are the four watches of the night, according to the Roman system, of three hours each, running from 6:00 P.M. to 6:00 A.M.

opse (cf. 11.11) ‘the evening,’ ‘late’: here, the first watch, from 6:00–9:00 P.M.

mesonuktion (only here in Mark) literally ‘the middle of the night’: here, the second watch, from 9:00 P.M. to 12:00 midnight.

alektorophōnias (only here in the N.T.) ‘at the crowing (phōnē) of the rooster (alektōr)’: here, the third watch, from 12:00 midnight to 3:00 A.M.

prōi (cf. 1.35) ‘early,’ ‘at dawn’: here, the fourth watch, from 3:00–6:00 A.M. (cf. 6.48 where ‘the fourth watch’ is referred to).

Translation:

Watch is variously translated in this context, e.g. ‘be on the look-out,’ ‘keep a close watch,’ ‘be constantly alert,’ or ‘wait, look.’

The paraphrastic expression included within dashes in the Revised Standard Version text combines quite well with the introductory verb watch. Where the difficulty arises is the connection of this sentence with the following verse, which expresses a negative purpose. This latter expression cannot be combined with the last of verse 35. There are usually three different types of solutions: (1) punctuation as a parenthetical expression, either within parentheses or dashes – a solution which is quite acceptable to rather literate people; (2) the repetition of ‘watch’ at the beginning of verse 36, so that the proper syntactic relationship may be preserved; or (3) an alteration in order in which the clause within dashes in the Revised Standard Version is placed first in the verse, followed by the imperative, ‘watch,’ e.g. ‘since you do not know when the owner of the house will come, whether in the evening, or at midnight, or at cockcrow, or in the early morning, therefore watch, lest he come suddenly and find you sleeping.’

Many languages have highly developed expressions for periods of the night, some of which rather closely parallel the series occurring in this verse. For example, the evening is ‘the sun lost’ (Tzotzil) and ‘the sitting together time’ (Mende). The period around midnight is ‘the stomach of the night’ (Uduk). A cockcrow is ‘when the owls make a noise,’ ‘when the cock screams’ (Piro), and ‘rooster-cry-time’ (Maninka). The period in the morning before sun-up may be described as ‘when the world begins to get white’ (Tzeltal), and ‘before the sky-opens-door’ (Maninka).

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .