In Access 2003 (but not 97), you can use conditional formatting to display
weekend dates in a different color.

In form design view, select the text box containing the date.
Choose Conditional Formatting on the Format menu.
Enter:
   Expression Is ...        Weekday([StartDate],7) <= 2
and click the color you want.

To do the same for public holidays, you would need a table of the public
holidays observed in your state/country. You could then use a DLookup()
expression in conditional formatting to see if the date is a holiday.

If this is a continuous form, or you have lots of date boxes to apply
conditional formatting to, you will notice the delay as Access performs the
DLookup()s.