Any information for the Season of Mastery open beta would likely be posted on the Classic General Discussion forum.
I’m afraid Customer Support doesn’t have any involvement in beta/ptr testing or have information on the schedule outside of what is already up. If the start of beta for the Season of Mastery is listed as today it likely will be some time today, but it could be several hours from now.
If I come across any other specifics I’ll be happy to post an update but I’d recommend keeping an eye on the Classic General Discussion forum, they’ll probably post there first.