Starting with "Iron Man" in 2008, Marvel has consistently opened its April/May film releases a week or two earlier overseas rather than in the United States first.
That is set to continue with the studio now confirming that "Avengers: Age of Ultron" is set to release in the United Kingdom and Ireland (and likely other overseas territories) on April 24th 2015 - one week ahead of the U.S. release on May 1st 2015.
Marvel's earlier openings overseas have proven a successful formula, leading to a few rivals copying the trend in recent years - albeit only in the same April/May release window (eg. "Star Trek," "Battleship," "Fast Five," "The Amazing Spider-Man 2") - a time when various countries are often in the midst of profitable Easter and/or school-related holidays.
It has also lead to the growing perception that American blockbusters are all opening overseas earlier than they are domestically, even though (with only the rarest exception) this remains only the case around the April/May release window and even then only for some titles.
Source: The Los Angeles Times