I'm fairly new to Access 2003, so apologies if this is a daft question.

In the design view of a new table i'm creating, i'm trying to set my TimeSlot field to ddd hh:nn format. For example, "Mon 21:30".

However, when I actually try to enter a date in the table, I get an error saying that it doesn't follow the format of the field. The same goes for entering a default value for the field back in Design View.

Help me please!

You shouldn't enter data directly into tables, therefore setting a custom date format isn't required.
Set it to general date/time and use forms to enter the data.

A date format has nothing whatsoever to do with how a date is stored. Although you can set a format to display only part of a date, the input requirements are less flexible. You are going to have to enter at least month/day. And if you also want to enter time, you'll probably need to enter all three parts of a date since the time part follows the date part.