The Space Devs run Launch Library, a REST API covering orbital and suborbital launches worldwide, past and upcoming. No key, no signup, and a forward schedule that currently runs to 369 launches. If you need a dated calendar of something genuinely interesting, this is one of the easiest open APIs to consume.

The Endpoints

Base URL carries the version in the path:

https://ll.thespacedevs.com/2.3.0/

The three that matter:

launches/upcoming/     everything scheduled ahead
launches/previous/     everything flown
launches/              both
events/upcoming/       spacewalks, dockings, test fires

A minimal call: