Option B) Consider Monday your 'fancy underwear' day and treat yourself a lacy pair reserved only for the start of the week (it's all downhill from there anyways), and discard the extra Thursday pair - out of sight out of mind.
Option C) Pretend Mondays do not exist - Peter Alexander is clearly of this opinion anyways - and simply live your life like it's almost the weekend, in a pair of Thursday undies on Monday. (You may need a therapist's help with this one though.)
Option D) Purchase another set of 7-day undies in order to get the missing Monday. Of course you'll have to discard the two extra Thursdays along with the extra Tuesday, Wednesday, Friday, Saturday and Sunday otherwise that part of you that needs structure and balance just won't cope.