what you do is this.

You get a pair of scissors and cut out a picture of your mum. Then chop a hole
in the middle so you can see what day  of the month it is.
Then simply get some selotape and stick the edges down along the sides to the
calendar.

Probably with DayStyle CSS style. If that doesn't go, try OnDayRender Event and
attach an attribute to each Calendar day.

I am sure this will look much better
though:
For those of you interested you add  a style to the day  you want in your
Calendar_Dayrender event like this:

Dim c as TableCell

C = e.Cell

c.Style.Add("BACKGROUND-IMAGE",
"url(http://localhost/examples/Calendar/Images/holiday_cal_test.gif
<http://localhost/examples/Calendar/Images/holiday_cal_test.gif> );")

if you are in code you can use C.CssClass = ds["styletitle"]

.... that way you could update the background  along with anyother cosmetics.
i.e. just alter the style.css and/or the database/xml which spits somestring

the overall client html's a bit easier to understand and less of it aswell ....
just a suggestion.