Trade deadline

A trade deadline,

(or trading deadline) in professional sports is a rule regulating the trading of professional players' contracts between clubs. Usually players acquired through trade after the trade deadline are ineligible for postseason play in that season, unless the respective league allows them to replace an injured player on the roster. In some leagues, post-deadline trades may be banned entirely. This term is used mainly in North America. In the approach to the deadline, there is heightened activity and interest in trades.

MLB

The Major League Baseball non-waiver deadline is July 31. After the All-Star break, teams will determine whether or not they are in position to contend for the post-season. Because of free agency and the lack of a salary cap in baseball, players in the final year of their contract are often put on the "trading block" by many of the non-playoff contending teams. Smaller market teams will not — or cannot afford to — pay their better veteran players high salaries, so they will attempt to trade them to a post-season contender, in exchange for some minor-league prospects or other players who might be able to help them in the future.

The MLB waiver deadline is August 31. There is much less activity between July 31 and August 31 because players must clear waivers. 

National Basketball Association

The National Basketball Association's deadline falls on the 16th Thursday of the season (usually in February) at 3pm Eastern Time.

NFL

The National Football League's deadline is the Tuesday following the sixth week of the regular season, which typically falls in mid-October. However, there is fairly little activity on this day and almost no star players are ever dealt to other clubs on the sixth week of the season.

Of the players who are dealt, usually they are wide receivers or defensive players so it is easier for them to learn the playbook.

NHL

NHL trade deadline

The National Hockey League's deadline is typically early or mid March.

Association football

Association football clubs have two transfer windows per season. In most countries, one falls during the middle of the season and the other runs during the close season (in Europe) or off-season (in North America). For those countries with seasons starting in August/September such as most European countries, the midseason transfer window usually falls in January, and the other one opens from June to September. In countries with seasons that operate entirely within a calendar year, like the leagues in Latin America or Northern Europe, the midseason transfer window opens in July/August, while the other starts from March to April. The difference between a transfer window and a trade deadline is that players cannot be bought or sold outside these transfer window periods. The notion of transfer windows was initially introduced in Europe, and subsequently adopted by FIFA. Like North America's trade deadline, there is greatly increased activity and interest as the close of a transfer window draws near.

Source:MLB Trade deadline