In a 5-4 spit decision The supreme Court upheld a Mississippi law that counts ballots received up to 5 days after election day as long as they are postmarked on or before election day.
The RNC had filed the case arguing the law is unconstitutional because federal law sets election day, not the states.
The Court ruled against the RNC. Allowing the grace period to remain in place.
Justice Amy Coney Barrett wrote the opinion.
"The [federal] election-day statutes require the electorate's choice to be made on election day. That occurs so long as election day is the deadline for individuals to vote—as it is in Mississippi," Barrett wrote. "But the election-day statutes do not set a deadline for ballot receipt, so they do not prevent Mississippi from counting ballots postmarked before election day yet received afterward."
https://www.supremecourt.gov/opinions/25pdf/24-1260_g3cn.pdf