When you experience a period of relative peacefulness before a time of upheaval or stress, you might look back on that peaceful period as the calm before the storm.
The phrase the calm before the storm is often used ominously, especially in hindsight, suggesting that an unusually tranquil period foreshadowed a difficult event, such as a war. But it can also be used in situations where someone is enjoying a quiet moment before a rush of activity, like a parent taking a nap before their twin toddlers wake up from theirs! The idiom comes from the literal phenomenon that sometimes occurs before a big storm, when the wind drops, the birds stop singing, and everything feels almost too quiet, right before the storm unleashes its fury.