Generally, probably as you are already aware, you can achieve this requirement by adding a SpecialDay to the Calendar of the picker control:
When the picker is placed within a grid template, you can use the same approach.
When the picker is generated automatically by the grid, you add the SpecialDay to its SharedCalendar instance.
First, you can access it during ItemDataBound:
Then, create a new special day and add it to the SharedCalendar of the picker, not its Calendar instance.
If ItemDataBound is too late, you can do this logic in the ItemCreated event handler:
I hope this will prove helpful. Give it a try and let me know if it works for you.
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