Complex sentence: It will be the Mets' first appearance in the World Series since 2000 when the Mets played against the New York Yankees who won in five games in a so-called Subway Series or cross-town matchup.
Split into simpler sentences: It will be the Mets' first appearance in the World Series since 2000. In 2000, the Mets played against the New York Yankees who won in five games in a so-called Subway Series or cross-town matchup.