One way:

Formula:

=MOD(A1-DATE(2007,1,11),14)=0

In article ,
Kaye wrote:

Hi,

I'm having difficulties setting up DV, using the Custom Formula, to
display the error message "Not a pay day" if the user enters a date
that's not a pay day.

Pay days are ....
Every second Thursday starting from 11-Jan-07, i.e.
11 Jan 07 + every 14 days.

Thanks, Kaye