Not many people know this, but Mastodon has a built-in official post scheduling system which can publish posts at pre-arranged times and dates.
Unfortunately there's no official interface for the scheduler yet, so you have to use a third party app to access the scheduler. These third party apps let you use it:
Android: Tusky, Fedilab, Moshidon & Subway Tooter
iPhone/iPad: Mona
🌐 Web: https://plan.fedilab.app
More info at https://fedi.tips/scheduling-posts-on-mastodon