Conn. Gen. Stat. § 12-39a

(b) For purposes of the timely performance of any act which is required or permitted to be performed, by a taxpayer or by the Department of Revenue Services, in connection with (1) the tax imposed under chapter 229, (2) the tax imposed on individuals who make purchases of services or tangible personal property, the storage, acceptance, consumption or other use of which is subject to the use tax under chapter 219, who have not paid the use tax due to any retailer required to collect the tax, and who make such purchases for personal use or consumption in this state, and not for use or consumption in carrying on a trade, occupation, business or profession, and (3) the tax imposed under chapter 228c, “legal holiday” includes any legal holiday, as defined in Section 7503 of the Internal Revenue Code of 1986, or any subsequent corresponding internal revenue code of the United States, as from time to time amended.

See Sec. 12-169 re payment date for local taxes due on Saturday, Sunday or legal holiday.