Senator Dianne Feinstein died in September 2023, but her term doesn't end until January 2025. When she died, the governor of California appointed Laphonza Butler to fill her seat in the senate until the next regularly scheduled election, at which point the voters would get to select someone to fill out the remainder of Feinstein's original term. (Note: Laphonza Butler is not running in either senate race.)
Here we are at the next regularly scheduled election, so we have two U.S. Senate races to vote for: