The Full Form of D DAY is‍ Designated Day.

designated day means a day of the week designated by the Remarketing Agent (a) in connection with a change in Mode as a day on which a particular action is to occur, or (b) as the first day of an Interest Period. It is recognized that different days of the week may be “Designated Days” for different actions. The Designated Day for the Series 2012A Bonds during the Weekly Rate Period shall be Thursday of each week, commencing with the earlier of the date of issuance of the Series 2012A Bonds or the first Thursday that is at least five days after the applicable Mode Change Date, or such other day as may be established by the Remarketing Agent with the consent of the Trust and the Credit Facility Issuer (if any) in connection with the establishment of that rate period.