My apologies, I have no idea why I thought this is about the grid :)
For selecting ranges in a calendar, we have the DateRangePicker component https://demos.telerik.com/blazor-ui/daterangepicker/overview
You could also use the range selection mode of the calendar too (see the third one): https://demos.telerik.com/blazor-ui/calendar/selection
With the multiple selection mode, you can use the ValueChanged event and get the currently selected date, store that and the next time the user changes it, you can compare it to the old value, see if it is later (or satisfies any other business logic), and then populate the SelectedDates list so that it contains all dates between the start and end you want. You can find some examples of handling that event here so you can start adding logic there.
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