Herbert Weir Smyth, A Greek Grammar for Colleges

a. To words denoting time when there is no attributive: ““ἐν τῷ χειμῶνι” in winter” X. O. 17.3; cp. 1444. b. When the attributive is a pronoun (sometimes): (ἐν) ἐκείνῃ τῇ ἡμέρᾳ. c. To statements of the time within the limits of which an event may take place (where ἐντός with the genitive is common); to statements of how much time anything takes; with numbers, ὀλίγος, πολύς, etc. Thus, ἐν τρισὶν ἡμέραις for (during) three days X. A. 4.8.8, ““οὐ ῥᾴδιον τὰ ἐν ἅπαντι τῷ χρόνῳ πρα_χθέντα ἐν μιᾷ ἡμέρᾳ δηλωθῆναι” it is not easy to set forth in a single day the acts of all time” L. 2.54, ““ἐξελέσθαι τὴν διαβολὴν ἐν οὕτως ὀλίγῳ χρόνῳ” to clear myself of calumny in so brief a time” P. A. 19a. ἐν is rarely omitted in prose, and chiefly when there is an attributive: μιᾷ νυκτὶ T. 6.27. d. Always with adjectives or adverbs used substantively: ἐν τῷ παρόντι, ἐν τῷ τότε. e. To words denoting the date of an event, not a point of time: ““ἐν τῇ προτέρᾳ πρεσβείᾳ” in the first embassy” Aes. 2.123. Thuc. employs ἐν, as ἐν τῇ ὑστεραίᾳ ἐκκλησίᾳ in the assembly held the day after 1. 44, but usu. the simple dative, as μάχῃ in the battle 3. 54, ἐκείνῃ τῇ ἐσβολῇ in that incursion 2. 20, τῇ προτέρᾳ ἐκκλησίᾳ in the first assembly 1. 44.