If the only reason you want the membership is to use it for the Mother's Day party, I can't recommend that you get it on the off-chance that you'll be allowed to use it on a blackout date, no matter whether or not another party may end up with a less alert server or manager (managers are required to approve all TIW discounts. It is not likely that a manager who follows the rules will give you a break just because you have a large party). If you're going to make use of the membership on other occasions, then you might still be okay with purchasing one and trying it.
I have been turned down for using the card for a beer at Rose & Crown on the 4th of July (handed it over automatically without remembering it was a blackout day, and the bartender handed it right back) so I can't say you will get lucky or you won't. You have one person here who had the discount applied on a blackout date, all others state they have been denied.