Markus 13:33

13:33 Watch out! Stay alert! ¹  For you do not know when the time will come.

Matius 24:42

24:42 “Therefore stay alert, because you do not know on what day ²  your Lord will come.

Matius 24:44

24:44 Therefore you also must be ready, because the Son of Man will come at an hour when you do not expect him. ³ 

[13:33]  ¹ tc The vast majority of witnesses (א A C L W Θ Ψ Ë^1,13 Ï lat sy co) have καὶ προσεύχεσθε after ἀγρυπνεῖτε (agrupneite kai proseucesqe, “stay alert and pray”). This may be a motivated reading, influenced by the similar command in Mark 14:38 where προσεύχεσθε is solidly attested, and more generally from the parallel in Luke 21:36 (though δέομαι [deomai, “ask”] is used there). As B. M. Metzger notes, it is a predictable variant that scribes would have been likely to produce independently of each other (TCGNT 95). The words are not found in B D 2427 a c {d} k. Although the external evidence for the shorter reading is slender, it probably better accounts for the longer reading than vice versa.

[24:42]  ² tc Most later mss (L 0281 Ï lat) have here ὥρᾳ ({wra, “hour”) instead of ἡμέρα (Jemera, “day”). Although the merits of this reading could be argued either way, in light of the overwhelming and diverse early support for ἡμέρᾳ ({א B C D W Δ Θ Ë¹³ 33 892 1424, as well as several versions and fathers}), the more general term is surely correct.

[24:44]  ³ sn Jesus made clear that his coming could not be timed, and suggested it would take some time – so long, in fact, that some will not be looking for him any longer (at an hour when you do not expect him).