one day - a day altogether unique, different from all others [Maurer]. Compare “one,” that is, unique (Son 6:9; Jer 30:7). Not as Henderson explains, “One continuous day, without night” (Rev 22:5; Rev 21:25); the millennial period (Rev 20:3-7).
known to ... Lord - This truth restrains man’s curiosity and teaches us to wait the Lord’s own time (Mat 24:36).
not day, nor night - answering to “not ... clear nor ... dark” (Zec 14:6); not altogether daylight, yet not the darkness of night.
at evening ... shall be light - Towards the close of this twilight-like time of calamity, “light” shall spring up (Psa 97:11; Psa 112:4; Isa 30:26; Isa 60:19, Isa 60:20).