The two upcoming series with the Yankees and Red Sox will be important because the six games will bring at least 1/10th of the season's total attendance through the Coliseum's turnstiles. The A's will have two other large-grossing series, June 24-26 vs. the Giants and September 2-4 vs. the Yankees. If the A's are in contention through August, they should be able to hit the 2.1 million figure at the very least. If not, they could drop below 2 million easily. Last season's 2.2 million is slightly off from 2003, and that small (1%) drop can be attributed to the A's losing the division on the 2nd-to-last game of the season.
Currently through 15 dates, the A's total is 309,117, with an average of 20,608 per date. That's down almost 108,000 from 2004, but through 15 dates in 2004, the A's had already played 3 games with the Yanks. This season's drop can be largely blamed on the poor weather seen so far this spring.
It'll be interesting to see how much the agressive season ticket marketing campaign has affected season tickets sales. This was considered Lew Wolff's #1 goal prior to getting a stadium deal in place. Even if the team drops in attendance due to poor performance, an expanded season ticket roll will be a very encouraging sign for the ownership group and peripheral investors as well.
Incidentally, the Giants' current total attendance is 758,618 through 20 dates, with an average of 37,931 per date. Not quite a sellout every game, but they're still on track to 3 million.