Thank you for writing.
In order to change the header font of the days in the day view, you should to apply the new font in the theme that you are using. You should update the theme with the new fonts for the SchedulerHeaderCell element and apply the new theme to the RadScheduler. Look at the attached screenshot to see what you should to change in the theme.
In case that you have other related questions, feel free to write back.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!