All ROOT-related meetings ca be found in ROOT's Indico category; the upcoming ones can be seen here to the right.
Developers get together on a weekly basis to discuss the plans and review the progress on Monday's at 16:00 CET.
Experiments and core developers meet regularly to discuss the progress of open issues and future plans on Wednesdays at 16:00 CET.
Several of the ROOT packages have dedicated meetings, for instance I/O, TMVA, RooFit, C++ Modules. These meetings, too, can be found in ROOT's Indico category.