Hello Vinay,

Unfortunately, UltraDateTimeEditor doesn’t have a functionality to highlight specific dates in the dropdown calendar.
Instead, I suggest you take a look at the sample posted by Mike in the forum below.

https://www.infragistics.com/community/forums/f/ultimate-ui-for-windows-forms/119955/need-input-mask-for-ultracalendarcombo

The sample in the above page(UltraDateTimeEditorWithUltraMonthViewMultiDropDown.zip) demonstrates how to create a custom UltraDateTimeEditor control that uses UltraMonthViewMulti as its dropdown calendar.
You should be able to attach UltraCalendarLook to UltraMonthViewMulti and change the background for whatever dates you like from there as Mike mentions.

In the attached sample for example, I setup UltraCalendarLook and change the color for 2019/5/1 and 2019/5/2 at CreateMonthViewMulti() in MyUltraDateTimeEditor.cs.

UltraDateTimeEditorWithUltraMonthViewMultiDropDown_1.zip