The Federal Election Commission has compiled primary dates for 2020. The chart shows the presidential primary date and also the congressional primary date. The FEC notes that these dates could still change.
Footnote 2, concerning Louisiana congressional elections, says, “In Louisiana, a Congressional primary is not held.” This shows that the FEC agrees with my commentaries about election systems. I have written many times that Louisiana does not hold primaries (except presidential primaries) and if a state wants to dispense with party nominees, then it is free to imitate Louisiana and simply abolish primaries. Supporters of top-two systems should take note of the Louisiana model. Certain supporters of top-two, when they do acknowledge that point, sometimes insist that Louisiana does hold primaries in November; but in reality, Louisiana elects people in November (to Congress), and thus there is no primary.
Thanks to Thomas Jones for the link.