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.
