I have a local post office, with notice in window, if shop is open so is the post office, so they are open 7 days a week, from 7am to 10pm.
So it is possible to post on a Sunday, but it is not your problem. You just need to receive your purchase, if not you open a not received case to get a refund.
In any case using ebay’s systems, items are marked as posted, when label is printed.