Mastodon has a built-in post scheduling feature, but it lacks an official interface so you can only use it through third party apps/services:
- Tusky, Fedilab, Moshidon and Mona support scheduling posts. Also a web-based service: https://plan.fedilab.app
- Professionals can use scheduling services like https://publer.io & https://buffer.com/mastodon
- If you're techy, you can self-host Mastodon Scheduler (https://github.com/scidsg/mastodon-scheduler) or Fediplan (https://framagit.org/tom79/fediplan)