Quiet hours
One schedule for the whole account. What arrives inside it is held, never dropped: in the inbox at once if you open it, silent otherwise, and delivered together when quiet ends.
The schedule
A from time and a to time in your local time, and the days it applies to. A day you leave out is quiet all day, which is how weekends go quiet. Off for a new account; the inbox suggests it once, the morning after the first night with three or more notifications, and never again after you answer.
What is held
A notification whose effective Reach would alert (Banner or Break through) from a source that respects quiet hours. It lands in the inbox as usual, shows no banner, plays no sound and does not count on the badge until quiet ends. Badge and Silent notifications are not held, since they never alert.
Each source decides for itself whether it respects the schedule; a new source does. Watches respect it per watch.
Critical
A critical notification breaks through quiet hours only when two switches are on: the source's "critical breaks through" and the account's master switch under Quiet hours. With either off, it is held like anything else at its Reach.
When quiet ends
Everything held is released at once: the badge counts it, and one summary push says how many arrived ("4 arrived overnight"), with one line for the summary. In the inbox the released notifications sit under their own header for 24 hours, "Held overnight · 4", with Mark all read beside it.
Email, Slack and webhooks
Held means held everywhere: an email, a Slack message or a webhook for a held notification goes out when quiet ends, not when it arrived.