From SA Mafia Wiki
The Hammer is the last vote needed to execute a player, usually joined with a :redhammer:. When the "hammer falls," typically the day is considered to have instantly ended - thus as per the standard mafia rules all posting should stop until the mod returns. If there is a Twilight phase, it usually occurs after the mod has posted the flip, but sometimes it means players can still post after the hammer. If there is a Governor role, between the hammer and the flip is when they need to decide whether to commute the execution. If the executed player is a Vengeful Townie, they also have this time to decide whom they would like to kill.
An Executioner can cause an execution to occur before hammer falls.