Most big stores don't do exchanges/returns/refunds on Boxing Day for obvious reasons. Walmart seems to have a very flexible exchange/return/refund policy (I returned an opened, used-once vacuum cleaner once because I was unhappy with its performance), and I would expect it to includes items purchased in the last 30 days as a minimum.
I don't know for sure about this year but in the past Walmart has done returns on the 26th. They must be the only retailer that does it and they have it down to a science.
I also, have been trying to find out what time they open...with no luck. My guess if 7am but I know they are open by 8am. I was in there last year at 8:15am.