Checking it seems "The Eleventh Hour" aired on 3 April 2010 on BBC and April 17, 2010 on BBC America. In other words, two weeks later.
This continued, until Memorial Day Weekend, when there was no new episode on in the US on May 29/29 May in the US, but "Cold Blood" aired in the UK. Starting June 5/5 June, there was now a 3 week difference in the air dates between the episodes airing in the UK and then in the US.
On June 26/26 June Britain aired "The Big Bang" and the United States aired "Vincent and the Doctor." Then the US took another week off for the July 4th/4th July. The new episodes resumed on July 10/10 July and the US got "The Big Bang" on July 24/24 July.
BBC America claims "A Christmas Carol" aired on Christmas Eve, December 24/24 December, but I remember it airing on Christmas Day.
As for Season 6, the UK and the US got "The Impossible Astronaut" on April 23/23 April. The episodes aired the same day until "The Almost People" on May 28/28 May, when BBC America did not air the episode due to Memorial Day Weekend. he US got the episodes a week later than the UK for "The Almost People" and "A Good Man Goes to War."
The second half of the season started on August 27/27 August for both the UK and the US. The rest of the season aired the same day in the UK and the US, without a break for Labor Day in the US (apparently, Memorial Day warrants a break in new episodes, but Labor Day does not).
"The Doctor, the Widow, and the Wardrobe" both aired on Christmas Day 2011.