Several project teams have already stated their intention to join the Eclipse Photon release. I've created a record for the release and have populated it with the information that's been provided to-date.
Please have a quick look.
If your project is listed, but does not have a release name beside it, that means that either a release record does not yet exist or that I could not sort out the version that you intended based on your message. If this is the case, please create the release record and let me know (you can do this off list).
Instructions for joining the simultaneous release are captured both in the simultaneous release documentation and in the Eclipse Project Handbook.
The actual deadline is M4 which will be sometime in December. Earlier is better than later.
FWIW, the Eclipse IP Team's prioritisation algorithm takes planned release dates into consideration. Creating reasonable and accurate records for future releases helps.
Director of Open Source Projects
The Eclipse Foundation