A public auction for Treasury bills that is held weekly by the U.S. Treasury. Currently there are 17 authorized primary dealers that are required to bid directly upon each issue. This is the manner in which all U.S. Treasury bills are issued.
(Read more: Bill Auction Definition | Investopedia http://www.investopedia.com/terms/b/bill-auction.asp#ixzz4P2dIZ536)
Marketable securities can be bought, sold, or transferred after they are originally issued. The U.S. Treasury uses an auction process to sell these securities and determine their rate or yield.
Treasury securities will be auctioned by viewing the announcements of pending auctions.
(soure: https://www.treasurydirect.gov/instit/auctfund/work/work.htm)