Usually you can't... Retail usually can't unless the broker screwed up your order and they they try to do a t or z trade. But if you have an unusual order (very odd size or other instruction) which you brokered w/ an MM, then you could (assuming the contract asked them to do it). Else it's damn near impossible.
But... TAs have been known to execute AH but that's some kind of arrangement w/ a MM which I don't understand too well.
falcons9, I'm with TDAmeritrade, too... As SoapyBubbles explained, there may be certain circumstances in which you can buy AH. However, you can buy AH if you want, but the order will not be executed until the next day, not during the AH time period. I tried that a couple of weeks ago AH on a Friday. It didn't execute until the following Monday. :)
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