Holiday rename; Juneteenth `start_date`
- [x] closes #44574

Rebased to origin/master.  Fixes #42328 and #44574.  Not sure if this is the right way to add to someone else's (closed) PR though...

> Thanks for picking this up - as requested in the original PR, could you add a test please?

Yep, on it, plus fixing tests that broke. I don't think I changed anything that caused the "Test experimental data manager (not slow..."  failure.  Is there a way to re-run this one step? It looks unrelated, don't worry about it Thanks @Bklyn! CI failure was unrelated. 