The broker will liquidate the position when the margin clerks get around to it, but you're better off being proactive.
If you've got a small account and get assigned a 10 lot position of SPY (which is 160k $) - you can probably close on Monday without a big change, but what if Monday AM the market is down 3% or up 3% - now you're looking at a 5,000$ change in value...
Always best to close early IMO - spend that nickel and sleep safe at night.
