National Signing Day is the first day high school graduates can sign a National Letter of Intent. The National Letter of Intent (NLI) is a binding agreement between a prospective student and an NLI member institution.
Although all sports in the NCAA Division I and II have at least one National Signing Day, college football's signing day usually receives the most attention in the sports media. Traditionally, National Signing Day in college football is the first Wednesday in February. This year it was February 2nd.
Student athletes are not required to sign their NLI on the first day of the signing period, but many do. Others don't receive and sign an NLI until after the first signing date. Many prospective students need more time to weigh their options before making such an important decision, or they wait to see if they get a better offer. However, this can be a gamble, as they may receive better scholarship offers, but they may also lose previous offers.
Sources: https://www.ncsasports.org/recruiting/managing-recruiting-process/national-signing-day, https://en.wikipedia.org/wiki/National_Signing_Day