All major banks are closed on Memorial Day. This also includes credit unions, the US post office, and the US stock market. You can make a deposit at an ATM on Memorial Day, but it will not be considered a business day and the transaction will not post until the following day.
Memorial Day is a federally recognized holiday and a day for mourning for any US service members who have died while serving in the military. We started formally observing the holiday in 1868 and it falls on the last Monday of May. Memorial Day also marks the unofficial start of summer.