Nov 26 (Reuters) - The following is a list of scheduled U.S. agency bill sales from Fannie Mae and Freddie Mac for 2012. Freddie Mac said 3-month and 6-month bills will be auctioned every week. In addition, 1-month and 12-month auctions are optional each week. Fannie Mae said 3-month, 6-month and 1-year may be auctioned on a weekly basis and it has the option to skip any bill auction. If it elects not to issue a scheduled offering, it will provide notice