The new Raspberry Pi OS with Debian 12 (Bookworm) will be released in the mit of October 2023.
https://www.raspberrypi.com/news/introducing-raspberry-pi-5/
A newer, better Raspberry Pi OS
In parallel with the final stages of the Raspberry Pi 5 programme, our software team has been busy developing a new version of Raspberry Pi OS, the official first-party operating system for Raspberry Pi devices. This is based on the most recent release of Debian (and its derivative Raspbian), codenamed “Bookworm”, and incorporates numerous enhancements, notably the transition from X11 to the Wayfire Wayland compositor on Raspberry Pi 4 and 5.
Raspberry Pi OS will launch in mid-October, and will be the sole supported first-party operating system for Raspberry Pi 5. Keep checking back here: we’ll be telling you some more about the new OS, and you’ll be able to download it shortly before Raspberry Pi 5 arrives on the shelves in late October._