California and Washington hold a nonpartisan blanket primary with the top two advancing to a runoff on election day even if a candidate gets 50%.
Louisiana holds the first round on election day and a second round after only if no individual candidate gets 50%.
But LA used to hold the first round in October with the second round on election day. However the Supreme court ruled this to be unconstitutional. Why is the California and Washington system allowed to still stand?
Because a candidate can win with 50+ in the first round of Louisiana elections and you can't win a federal office before federally mandated election day.
Altho, Congress does not have to mandate a particular national election day. Fun fact: Back when Congress normally held its first session some nine months after the official start of the term, several states waited until a month before the start of the session to have the election, unless something like a Civil War caused a special session to be called earlier than normal, in which case they quickly held special elections for the vacant seats.