Operation Nightfall is the name of the free update not a mode to enable.
It brings night versions of the maps to existing modes like coop checkpoint, hardcore checkpoint, Frenzy or time-limited modes (like only nightmaps when the update launched) and also for time-limited pvp mode once ever while. This means if you can vote for a night map or day map when playing one of these modes. When you play a night map the loadout is also for night operations, this means Nightvision, lasers, flashlight...
If you only look for nightmaps, you can go to the community servers and search for one with only night maps (they also offer really cool co-op servers with modded maps, hardcore ai, new loadouts... if you like this kind of content, then there are many clans around doing this and hosting these servers. Just join them.