From Ballotopedia
"Special elections to U.S. Congress are required in the event of vacancies.
As of April 2025, two special elections have been held, and there are two upcoming special elections scheduled to take place. The following special elections are upcoming:"
Arizona's 7th Congressional District on September 23, 2025
https://ballotpedia.org/Arizona%27s_7th_Congressional_District_special_election,_2025
Texas' 18th Congressional District on November 4, 2025
https://ballotpedia.org/Texas%27_18th_Congressional_District_special_election,_2025
See below for details about upcoming important elections throughout the U.S.