I understand that it seems pretty crappy but where do you draw the line? It's a one day sale on the internet for 50% off...it's not a seasonal sale. Let's say hypothetically that they honor the 8th day for you, so then they should honor the 9th day for the fellow after you since it's so darn close to the cutoff. They could run in parallel with the 30 day return policy but I'm not sure they would even bother with such a high mark off sale then...nothing personal, it's just business. I suppose they could do it on a case by case basis but with the traffic they get, probably would turn into a worse customer service nightmare. Did you get a good price in the first place? I assume you shopped around and they do have very good prices regardless.
Maybe I'm on my high horse...I bought my board and bindings from them last Thursday for a price I couldn't get anywhere else. So when I saw the sale not knowing the 7 day policy, I told myself that I bought my stuff for the best price by far at the time and plain crappy luck that I didn't get in on this. It was icing on the cake when I saw my account and the order history indicated that I was eligible for a price match resulting in an $80 gift card. Tomorrow I think I should be getting a $40 credit for the bindings.
Just tell yourself you got a good deal the first time around and it was worth it for your son.
If you want to know about missing something by one day...last year, my 03 Nissan 350Z needed a new motor due to a vacuum pressure problem. Unbeknownst to me, the extended warranty expired the day previous...they wanted to charge me $9500 for a new long block and Nissan North America would not budge over one F'n day. My car is sitting in the garage...been debating on rebuilding the motor with race parts (Cosworth stuff) or selling the damn thing to someone who wants to put the time and effort in. You don't want to give business to Sierra even though their prices are good and IMO so is their customer service (returns, shopping guidance, order tracking) because you want them to bend the rules over $32, I'm never buying another craptastic Nissan over a $9500 estimated charge for a 3rd motor (yes, it was going to be my 3rd motor...the first year Z's were known to have tranny and motor oil consumption issues) for the same reason...I wanted them to bend the rules in good faith.
It's your right and prerogative to give them business or not and as shitty as it is, I can see an equally, if not more so, compelling argument that no one is entitled to anything above and beyond a company's clearly stated policy. They didn't rip you off, you just missed out unfortunately.