In my opinion, the most elegant solution would be to take the dates from the template, so you could manage your data elsewhere. For instance:

[% USE MessageOfTheDay %]

[% MessageOfTheDay.Message %]
[download]

Data file:

New Year's Day      *-01-01
April Fools' Day    *-04-01
Independence Day    *-07-04
Christmas Day       *-12-25

Any given week      2006-04-09  2006-04-15
[download]

The actual code is left as an exercise for the reader.