Issue while configuring Repeating Event
I'm thinking the invalid day name error is the result of the start date of June 10, 2019 which is a Monday, not a Sunday (note that in your screenshot, there is no date listed for the "Original" event). If you change the start date to June 9 (which is the second Sunday in June), then the repeat function works as expected (on 5.10.alpha1):



Hope this helps,
Tamar