The article is often omitted (1) in words and phrases which have survived from the period when ὁ, ἡ, τό was a demonstrative pronoun; (2) when a word is sufficiently definite by itself; (3) when a word expresses a general conception without regard to its application to a definite person. The generic article is frequently omitted, especially with abstracts ( cross1132), without appreciable difference in meaning. Its presence or absence is often determined by the need of distinguishing subject from predicate ( cross1150), by the rhythm of the sentence, etc.
The article is omitted in many adverbial designations of time, mostly with prepositions (except ἡμέρα_ς
Thus, περὶ μέσα_ς νύκτας
The article is very often omitted in phrases containing a preposition:
ἐν ἀρχῇ τοῦ λόγου
Ἠιόνα τὴν ἐπὶ Στρυ_μόνι
Words denoting persons, when they are used of a class, may omit the article. So ἄνθρωπος, στρατηγός, θεός
πάντων μέτρον ἄνθρωπός ἐστιν
Adjectives and participles used substantively have no article when the reference is general:
τῶν ἐχθρῶν ἄρδην ὄλεθρος
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].