While the day was coming on (achri hou hēmera ēmellen ginesthai). More likely here achri hou (for achri toutou hēi) with the imperfect ēmellen, has its usual meaning, “until which time day was about to come on (ginesthai, present middle infinitive, linear action).” That is Paul kept on exhorting or beseeching (parekalei, imperfect active) them until dawn began to come on (cf. Act 27:39when day came). In Heb 3:13 achri hou with the present indicative has to mean “so long as” or while, but that is not true here (Robertson, Grammar, p. 975). See note on Act 2:46 for the same phrase for partaking food (metalambanō trophēs, genitive case) as also in Act 27:34. Paul wanted them to be ready for action when day really came. “Fourteenth day” repeated (Act 27:27), only here in the accusative of duration of time (hēmeran). It is not clear whether the “waiting” (prosdokōntes, present active participle predicate nominative complementary participle after diateleite, Robertson, Grammar, p. 1121) means fourteen days of continuous fasting or only fourteen successive nights of eager watching without food. Galen and Dionysius of Halicarnassus employ the very idiom used here by Luke (asitos diateleō).
Having taken nothing (mēthen proslabomenoi). Second aorist middle participle of proslambanō with the accusative mēthen rather than the more usual mēden. Probably Paul means that they had taken no regular meals, only bits of food now and then.