The Trade Deadline
One afternoon at the end of July after which no club can trade anybody for the rest of the season.
What it actually is. Clubs may trade players until a fixed date at the end of July, and after it no trades are permitted for the rest of the season, including the postseason. Clubs in contention buy, usually giving up prospects for players who help now; clubs out of it sell, usually trading players in the last year of their contract for prospects.
What somebody who knows the game gets out of it. The deadline is where an organization's honesty about itself becomes public. The expanded playoff field made the middle of the standings much more valuable, which reduced selling and pushed prices up, because more clubs believe they are in it. The consistent finding is that rental players are usually overpriced in wins added: a two month upgrade is worth around one win, and clubs regularly pay prospects worth several.
The numbers worth knowing. A club adding a two win player for two months gains roughly 0.6 wins. Deadline weeks typically produce 30 to 50 trades across the league.
What people get wrong about it. The August waiver trade period no longer exists. It was eliminated in 2019, so the July deadline is final, and a club whose starter gets hurt in August has whatever it has.
At Little League and travel ball. There is no equivalent, but the travel version of the same question, whether to add a guest player for a tournament, is governed by sanction rules about roster locks and guest eligibility, which the travel section of this site covers by sanction.
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