Sat/Sun works for me too.
I like the idea of posting to the hangout thread when you are online. Even better would be to receive a notification when someone is using the room.
I think that the mobile app has notification settings when a contact is in a hangouts call.
If you'd like to receive a notification each time someone in your Friends list becomes available online:
Click Settings at the top of your Friends list.
Highlight Notifications along the left side of the Settings window.
Select the box next to Show notification, under Friend Online.
This might be a good way to build up contacts for the sketch/study group.