Assuming that you are talking about the ASPNet AjaxToolKit Calendar Control:
The relevant class for the header is "ajax__calendar_header
". You override this class in your css.
.ajax__calendar_header { background-color: yellow !important; }
Edit: after op comments:
Using the server control, there is a property called TitleStyle
. You can specify your formatting here. Alternatively, you can use the property TitleStyle-CssClass
to apply a css class to the header.
<asp:Calendar ID="cal" runat="server" TitleStyle-CssClass="cssclass"></asp:Calendar>
Hope that helps.