On postback, to get the selected date from the calendar, use SelectedDate property.

By default, calendar doesn't automatically post back when date is selected.
To make it post back automatically, set AutoPostback property to true.

You can know when the selected date of the calendar has changed by defining
a DateChanged event handler. See examples below.

In this example, calendar doesn't automatically post back, but instead selected date is processed on button click.

Name:    DOB:      << Tuesday, May 30, 2023 >> 00 : 00 : 00 Set   00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23     00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59     00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

In this example, calendar automatically posts back when date is selected. Selected date is processed on DateChanged event.