Technically, a baseball trade is called a Trade Assignment (similar in kind to an Optional Assignment or Outright Assignment).
Generally, an MLB club can trade a player on its MLB Reserve List (40-man roster) and minor league reserve lists at any time. However, there are a few restrictions:
1. A trade involving a player signed to an MLB contract (including any player on an MLB Reserve List, MLB 60-day Injured List, or MLB Restricted List, and/or any player who was outrighted to the minors after signing an MLB contract for that season) is prohibited beginning at 4 PM (Eastern) on July 31st (or 4 PM Eastern on Friday July 30th if July 31st falls on a Saturday, or 4 PM Eastern on Monday August 1st if July 31st falls on a Sunday) and extending until the day after the final game of the World Series.