Seems like the days selected for a multi-day view are entirely wrong.
Fresh install of drupal 9, fresh install of calendar et al.
Add a single node with a data stretching over a weekend (e.g. Thursday till Tuesday)
Build a monthly view (default), showing the node...
Select multi-column row as the multi-day style - leave everything else a default.
View the calendar - and see the start date and end date are somewhat random, and always in the same week.
Am I using the module wrong, or is this just not supposed to work?
Needs review
1.0
MultiDay
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I rerolled the patch from @zero2one to 10.3.10 and made a MR for this.
Also gonna upload the MR diff here as a patch for ease of use.