April 3

From The Met Wiki

1989: The Mets defeat the Cardinals, 8-4, in the season opener at Shea Stadium.

2000: The Mets defeat the Padres, 2-1, at Shea Stadium in their first American-based game of the year. The team had officially begun the season with two games in Japan the previous week.

2001: The Mets defeat the Braves in 10 innings, 6-4, on Opening Day in Atlanta. Robin Ventura hits two 2-run homers, one of which is the game-winner.

2006: The Mets win the season opener, 3-2, over the Nationals at Shea Stadium.