When is NEXUS Blitz in??

Nexus Blitz has been cancelled for the foreseeable future, unfortunately. It's most likely only going to return for a limited time during some events.

To quote Riot directly:

"While we’re no longer considering Nexus Blitz as a permanent mode for the foreseeable future, we are going to treat it as a great mode to accompany events, especially given its flexibility to feature new in-game events and rewards. We’re already starting to think about when it could come back!"