SQL to return record counts in X intervals

SQL to return record counts in X intervals

I have a table like this:
id | created_on
1 2013-09-03 20:05:09
2 2013-09-05 17:03:13
...
How do I write a query to return a result of record counts that was
created from Date X to Date Y in 7-day intervals?
So the result would look like this:
count | created_on
4 2013-09-17 00:00:00
2 2013-09-24 00:00:00
1 2013-09-31 00:00:00
10 2013-10-07 00:00:00
...