Under Settings → Scheduling rules → Date exceptions, add the days that don’t follow your usual week, such as public holidays.
For each date, choose its Effect:
Deliveries and fulfilment: nothing is sent or delivered that day.
Deliveries only: you still fulfil orders, but nothing is delivered that day.
Fulfilment only: you don’t fulfil orders that day, but carriers still deliver, for example during a stocktake.
Changes shipping rates (Growth and Plus): customers who choose that date can pick only the shipping rates you tick, such as your Saturday rate on Christmas Eve. See Shipping rates by day.
And whether it Repeats: doesn’t repeat, every year (Christmas Day) or every month.
Before you save, a preview shows what closing a date changes, such as “Deliveries on Tue 22 Dec are fulfilled on Fri 18 Dec instead of Mon 21 Dec.”