You probably thought it yourself: Why are they playing the Subway Series so early this season?

Answer: Because that's what was on the schedule.

There is no rhyme or reason why the Yankees and Mets played three games in April -- with the series finale Sunday night at Yankee Stadium -- and will play another three Sept. 18-20 at Citi Field.

In the 19-year history of the regular-season Subway Series, there have been several format changes.

The Yankees and Mets played three games a season in 1997, when interleague play was born, and 1998. In 1999, the "natural rivalry'' series were expanded to six -- three at each team's ballpark.

The format was changed again in 2013 with each league having 15 teams and interleague play taking place nearly every day. The Yankees and Mets played a four-game home-and-home series on consecutive days in 2013 and 2014.

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This year, the teams went back to a six-game home-and-home because the NL and AL East teams are facing each other in interleague play. Next year, it's back to four games. Will they be on consecutive days again? That's up to the schedulemaker.

"I don't think there's a date you can pick and say, 'This is the best time,' '' Mets manager Terry Collins said. "It is interleague. Everybody's got to do it. Didn't keep the people from showing up, which is still fun. I don't care when you play it.''

The weather this weekend has been less than ideal, but more than 135,000 people still packed Yankee Stadium for the three-game series.

"This a lot of fun right now at this time of year,'' Collins said. "Because we're playing good. The city's excited. I just want the same atmosphere in September. If that's the case, then we've got something we can talk about.''

Said Yankees manager Joe Girardi: "Does it matter when we play them? From a scheduling standpoint, I don't think so.''

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Girardi may feel differently in September when the Yankees visit Citi Field and lose their designated hitter for three games.

Having to play without one of your best hitters is something that AL teams have had to put up with since 1997. But interleague play used to be concentrated in the middle of the summer and teams would play within their own leagues during the heart of the pennant race. That no longer is the case.

"I don't know how you avoid that with 15 and 15,'' Girardi said. "Because someone's going to be playing late-season National League [rules]. To complain about that probably wouldn't make any sense because you're going to have to do it at some point.''

One easy way to fix that would be for the NL to adopt the DH or the AL to drop the DH. But new commissioner Rob Manfred has said he has no problem with the leagues having different rules. It could be addressed in the next collective-bargaining agreement, but Manfred hasn't made it a priority.

The Yankees and Mets split the first two games going into Sunday night. While fans care about bragging rights, the Mets were more interested in continuing their red-hot start than figuring out how they stack up against the Yankees.

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"I don't ever try to compare my team to anybody's team,'' Collins said. "I've got my own guys. I know what they can do. To be honest, they've won 27 championships over here. There should be a lot of teams that can't compare themselves to them. It's about going out and doing what you have to do.''