thumb|right|Massachusetts [[British America|colonial Governor Jonathan Belcher's February 26, 1735 (NS 1736) proclamation of a fast day for April 1.]]
Fast Day was a holiday observed in some parts of the United States between 1670 and 1991.
"A day of public fasting and prayer," it was traditionally observed in the New England states. It had its origin in days of prayer and repentance proclaimed in the early days of the British American colonies by Royal Governors, to avoid such calamities as plagues, natural disasters or crop failures; it was common to hold a Fast Day before the spring planting (cf. rogation days). It was observed by church attendance, fasting, and abstinence from secular activities.
A Fast Day could be added for any particular reason in a particular year, rather than being instituted as an ongoing annual holiday. The earliest known Fast Day was proclaimed in colonial Boston, held on September 8, 1670. Maine, which also celebrated Fast Day on the third Monday in April, changed it to Patriot's Day<!--- Maine uses different punctuation, as per cited article ---> in 1907.