[CentOS] OT ? Shell script

Does anyone have an elegant solution for finding the date of the 3rd 
Monday each month and the date of the Sunday preceding?  (Sunday 
shouldn't be beyond my capabilities, once the Monday part is worked out.)
For the past several years, I've been handling the chore using a file 
with a manually input list of dates -- which is about as elegant as 
driving a tack with a sledge hammer.