From Scotusblog: Justices agree to decide major election law case

The case is Watson v. Republican National Committee

It concerns mail-in voting and whether they can be counted if receive after the election. 

Dear ChatGPT: What are the controversies concerning counting mail in ballots?

From the article: 

Setting the stage for a major ruling on election law, the Supreme Court on Monday agreed to decide whether federal law requires ballots to be not only cast by voters but also received by election officials by Election Day. As part of the list of orders from the justices’ private conference on Friday, the court took up Watson v. Republican National Committee, a challenge by the Republican National Committee and others to a Mississippi law (as well as similar laws in 30 other states and the District of Columbia) that allow mail-in ballots to be counted as long as they are received within five business days after Election Day. . . .