The first day of a new Congressional term in the U.S. Senate includes a variety of procedural activities, such as swearing-in of new or re-elected senators and electing new Senate officers including president pro tempore. The procedure for starting a new Congress is grounded in tradition and has been codified into a set of rules.
Much of the process of getting things done in the U.S. Senate is governed by a well-established traditional set of rules, and starting a new Congressional session, or starting a new Congress, is no exception. On the first day of a new session, senators starting a new term, whether it's their first term or the result of a re-election, have to be sworn in before they can perform any official duties. Senatorial officers have to be elected, including multiple positions if there has been a major party shift in the latest election. The Senate can then take care of some administrative details or dive right into voting on new legislation.