calm before the storm

the calm before the storm and
the lull before the storm (a peaceful and quiet period immediately before a period of great activity or trouble; a period of unusual tranquillity or stability that seems likely to presage difficult times) — затишье перед бурей

Example 1: The family are arriving this afternoon so I'm just sitting down with a cup of coffee, enjoying the calm before the storm. (CIDI)

Example 2: For most teachers, the days just before the school year begins are the calm before the storm. (Cambridge Dictionary of American Idioms)