Is there some way to select a range of days in TPlannerMonthView? Something like TPlanner.SelectCells would be great or even if you could change the background color of certain days. I want to highlight a date range from a search so OnGetDayProp doesn't seem appropriate.

I notice that SelectCells actually does not work for multi-selection in TPlanner. I also need to select multiple columns in TPlanner. Using SelectCells on multiple columns selects only the last column (position) passed to SelectCells.

TPlannerMonthView, using OnGetDayProp actually does seem to be workable.

As operations in the Planner can only be done on a single resource at a time, selection over multiple resources is not possible.
For TPlannerMonthView, the selected dates are handled via the collection PlannerMonthView.Dates

  PlannerMonthView1.Dates.Add.Date := now + 1;
This sets tomorrow as selected date.

Thanks. I don't really need to select resources. Any kind of highlighting would do. Is there no way to highlight or change the background color of multiple columns in TPlanner?

With Planner.OnPlannerBkgProp() you can dynamically change Planner cell colors and with Planner.BackgroundColor[col,row]: TColor you can set it via property.