calm before the storm

American  

[kahm bi-fawr thuh stawrm, kahlm] / ˈkɑm bɪˈfɔr ðə ˈstɔrm, ˈkɑlm /

Also lull before the storm

idiom

1. a lull or temporary period of quiet before a tumultuous event, such as a storm or an episode of excitement, activity, violence, etc..

   The evening was very pleasant, and it was surprising to see no one taking advantage of the calm before the storm.

   April felt like the calm before the storm as my calendar for May through July has very quickly filled up with concerts, visitors, and trips.

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Etymology

Origin of calm before the storm

First recorded in 1600–10