You can make posts appear on your Mastodon account at specific times using the post scheduling feature.
Scheduling isn't in the official apps or web interface, but it is available through third party apps such as Tusky and Fedilab, and through third party websites like https://plan.fedilab.app and https://buffer.com/mastodon
The feature is built into Mastodon, but for some unknown reason there's no official interface for it yet. That's why it is only available through third parties.
@feditips ah that's interesting. Since it wasn't on the official client I always assumed the other clients saved and sent the posts themselves at the right time but it sounds like they post them to the server with the publish date and the server makes them appear at the right time?
Yeah, the back end of the scheduling feature is built into Mastodon and provided to third party apps through the Mastodon API. There's just no official interface, so this feature is invisible to people who only use official apps or the official web interface.
If you're comfortable using github, you can vote for an official interface at https://github.com/mastodon/mastodon/issues/10088