The U.S. is different from most other democracies in how we run elections. One major difference is that, in most other countries, the local party committees (with some suggestions from the national party) pick the candidates. In the U.S. that only happens (and in only in some states) when there is a special election. Instead, the rule in most states is that candidates for the general election are chosen by partisan primaries.
And because there is no federal law governing the timing of primaries, it is up to the states to decide when they want to hold their primaries. But, because federal law does set the date of the general election for presidential electors, U.S. Senators, and U.S. Representatives (the first Tuesday after the first Monday in November) and also requires that absentee ballots e mad available by early October to military voters, the latest that a state can hold their primary (and canvass the ballots at the state level) while still having time to print general election ballots is around mid-September. So, particularly, in a presidential election year, primary season runs from February through September.
We are down now to the last four states to hold primaries. (Louisiana, technically, does not have a primary. Instead, they use a semi-nonpartisan general election on which all candidates from all parties appear on the general election ballot with a runoff several weeks later if nobody gets a majority.)
