The periods of highest liquidity in the stock markets are always during trading hours, usually right at the open and about ten minutes before the close to the closing bell. Many companies are so liquid that trades are placed near instantaneously throughout the day, but if you are invested in smaller companies, there could be a substantial lag between when you place an order and when it is filled. There may be no one on the other side of the trade, and that is compounded after-hours or pre-market, when liquidity is low.