Feb 24 (Reuters) - The following are scheduled sales announcement dates for potential Freddie Mac reference, Fannie Mae benchmark and FHLB global notes for 2012. Freddie Mac and Fannie Mae said the dates below are windows of optional note issuance. Freddie Mac and Fannie Mae said on each announcement date it will provide the maturity date and dealer syndicate, and that it may forgo any scheduled monthly issuance. FHLB said the dates below are windows of optional