February 15, May 15, August 15 and November 15, or next working day offerings of several "coupon" security issues. Quarterly issues currently consist of a 3-year note, a 10-year note, and a 30-year bond. The Treasury sometimes offers additional amounts of outstanding long-term notes or bonds, rather than selling new security issues. See: Reopening.
A debt security in which portions of the same issue are offered four times per year, on the 15th of February, May, August, and November. That is, rather than making a new issue, an issuer may use quarterly financing instead; this usually saves on the expense of making a whole new issue. Examples of quarterly financing include three-year and 10-year notes, as well as 30-year bonds.