With an odd number of teams in each league (15), it is necessary to have two teams participate in interleague play for most days in the season, except when two or more teams have a day off. Each team plays 20 interleague games throughout the season, usually with just one interleague game per day, but for one weekend in late May, all teams will participate in an interleague series. Use of the DH rule is determined by the home team's league rules. Before 2013, interleague play was structured differently: there would be one weekend in mid-May and another period consisting typically of the last two-thirds of June in which all teams played interleague games (save for two NL teams each day), and no interleague games were scheduled outside those dates. (Before 2013, season-long interleague play was not necessary, because each league had an even number of teams. In 2013, the Houston Astros moved to the American League, so that each league would have 15 teams.)