Over 1 billion IOS and Android apps were downloaded in the last week of 2011, according to Flurry, a company that tracks, analyzes and monetizes mobile apps.

In a recent blog post, the company said that Christmas Day through December 31 is always the busiest week of the year for app downloads. They point out that the week after Christmas “is far more important than the run-up to Christmas” for app makers.

Flurry also said that Christmas Day is the busiest single day for device (smartphones, tablets, etc) activations, with a recored breaking 6.8 million activated on Christmas Day 2011. Based on their analysis, Flurry estimates that over 20 million Apple and Android devices were activated from Christmas Day to December 31, 2011. And they estimate that 1.2 billion apps were downloaded during that same timeframe.

Looking deeper into their analysis, we find that most of the downloads occurred in the US, with approximately 509 million downloads. China, which they say is the world’s second-largest app market, came in number two with 99 million downloads. They were followed by the UK with 81 million, Canada with 41 million, and Germany and France with 40 million apiece. South Korea, Australia, Italy, Japan, and Spain also had more than 20 million estimated downloads.

Looking to the future, Flurry said they expect 1 billion download weeks to be more common-place as we get further into 2012.