The US market is open Mondays to Fridays from 9.30 am to 4 pm EST except on US trading holidays. The orders will be queued if placed outside market hours, except for limit good-to-day (GTD) orders which can be executed in extended hours (4 am - 9.30 am EST and 4 pm - 8 pm EST) as well.
All other order types will only be executed during market hours (9.30 am - 4 pm EST). 

Can I trade during the US exchange holidays?

The US stock market observes US public holidays. Any orders you make on these days will not be filled until US stock exchanges re-open.

Note: US public holidays do not count as a business day.
                       

Here is a quick summary of the supported time slots for different order types: