Two weeks ago I noted how the U.S. domestic box office has been outperforming on a year-to-date basis (YTD). Figures through mid July tally ticket sales at $5.9 billion, up 15% YTD.

Looking at the cumulative ticket sales a large growth occurred during the first part of the year. Up through the first “third” of this year (the first 17 weeks right before the summer season as of the first week of May), ticket sales totaled $3.1 billion, about 22% more than the same period last year.

However, looking at the just the summer, between weeks 18 and 28 (highlighted in yellow below), the performance has been less outstanding. For this period, 2009 ticket sales stand at $2.8 billion, about 7.8% more than last summer’s sales at the same point.

It is possible that Hollywood had a stronger first quarter because of stronger titles or stronger demand. In another post I’ve noted how few big budget films open up during the first quarter and how this may be an opportunity for Hollywood. During the beginning of the year there was speculation that the recession was creating a need for “escapism” and thereby driving greater movie attendance.

In any case it seems that attendance growth is dissipating and that the rest of the summer will continue along the same trend. Although this past weekend Harry Potter and the Half Blood Prince opened up strong to $77.8 million, this is half of the $158.4 million The Dark Knight made during the same weekend last year during its opening. Comparing total ticket sales for the top 12 performers during the same weekend, 2009 also undperforms 2008: $152.2 million to $253.6 million. Considering that The Dark Knight went on to be the number one movie in 2009, grossing $441.6 million domestically, it may be difficult for Harry Potter and the rest of the pack to keep up. Given that I don’t see any with true blockbuster caliber in the pipeline during the next seven weeks of this summer, it is probable that the lead 2009′s box office has over last year will lessen.