Your order will be processed in real time when the exchanges are open. Exchanges are typically open Monday through Friday from 9:30 a.m. ET to 4:00 p.m. ET and closed for observed holidays. If an order is processed after the exchange is closed, it will be processed at the beginning of the next trading day. For a list of observed holidays, see the NYSE and NASDAQ web sites.
If selling a stock or ETF, funds are available for immediate withdrawal or re-investment. There is a process called settlement that occurs two business days after your trade is processed. For this reason, you can’t buy and sell stocks using the same funds multiple times before settlement occurs.