Stock market hours are from 9:30am - 4pm US Eastern Time, Monday through Friday, excluding some holidays (check out the full list of “market holidays” here). You can still place orders to buy or sell stock while the market is closed – those orders will be completed when the market re-opens.
It’s important to know that even when market is closed, the world spins on! News and trading can still happen “After-hours”, meaning that the price of a stock can be different at market close one day than it is at market open the next. This is especially important when it comes to pending orders, since a stock you’ve placed an order for after the market has closed could cost more than you have in your Public account when the market re-opens.
There's quite a bit of information behind this, so check out our Guide to Understanding Stock Prices to learn more.