Question: John Sebastian is an American-born singer-songwriter, guitarist, harmonicist, and autoharpist,  best known as a founder of The Lovin' Spoonful, that did an impromptu appearance where in 1969? 

Context paragraphs:
Woodstock: The Woodstock Music & Art Fair—informally, the Woodstock Festival or simply Woodstock— was a music festival in the United States in 1969 which attracted an audience of over 400,000.  Scheduled for August 15–17 on a dairy farm in the Catskill Mountains of southern New York, northwest of New York City, it ran over to Monday,

Summer in the City: "Summer in the City" is a song recorded by The Lovin' Spoonful, written by John Sebastian, Mark Sebastian, and Steve Boone.

John B. Sebastian (album): John B. Sebastian is the debut album by American singer/songwriter John Sebastian, previously best known as the co-founder and primary singer/songwriter of the 1960s folk-rock band the Lovin' Spoonful.  The album, released in January 1970 (see 1970 in music), includes several songs that would become staples of Sebastian's live performances during the early and mid-1970s.  Most notably, the album included "She's a Lady", Sebastian's first solo single (released in December 1968), and an alternate version of "I Had a Dream" which was used to open of the 1970 documentary film "Woodstock".  "John B. Sebastian" also featured support performances by David Crosby, Stephen Stills and Graham Nash several months before that trio agreed to work together as a performing unit.

John Sebastian: John Benson Sebastian (born March 17, 1944) is an American-born singer-songwriter, guitarist, harmonicist, and autoharpist, who is best known as a founder of The Lovin' Spoonful, a band inducted into the Rock and Roll Hall of Fame in 2000; for his impromptu appearance at the Woodstock festival in 1969; and for his No. 1 hit in 1976, "Welcome Back".

You're a Big Boy Now (album): You're a Big Boy Now is a soundtrack album by The Lovin' Spoonful, released in 1967, containing music from the Francis Ford Coppola film of the same name.  Composed entirely by Spoonful member John Sebastian, it contains several songs performed by the band, as well as instrumental music from the film score.

Daydream (The Lovin' Spoonful song): "Daydream" is a popular song written by John Sebastian, published in 1966.  The song was originally recorded that year by Sebastian's group The Lovin' Spoonful and released on their album of the same title.

Even Dozen Jug Band: The Even Dozen Jug Band was founded in 1963 by Stefan Grossman (solo country blues and ragtime guitarist) and Peter Siegel (roots-based guitarist and producer) in New York City, New York.  Other members were David Grisman (a noted mandolinist), Steve Katz (later with Blues Project and Blood, Sweat and Tears), Maria Muldaur (then Maria D'Amato), Joshua Rifkin (arranger of Scott Joplin Ragtime compositions,), and John Sebastian (later with the Lovin' Spoonful).

Darlin' Be Home Soon: "Darlin' Be Home Soon" (or "Darling Be Home Soon") is a song written by John Sebastian of The Lovin' Spoonful for the soundtrack of the 1966 Francis Ford Coppola film "You're a Big Boy Now".  It has been described as "...one of the most heartfelt songs about being away from a loved one, written from the point of view of a musician on the road writing a letter."  It appeared on The Lovin' Spoonful's 1967 soundtrack album "You're a Big Boy Now" and was released as a single, reaching #15 on the US pop charts.  In the same year, a version by Bobby Darin reached #93 on the US charts, saxophonist Bud Shank put the track on his album "A Spoonful of Jazz", and Billie Davis put it on the B-side of a single in the UK.

No Favors (Big Sean song): "No Favors" is a song by American rapper Big Sean featuring American rapper Eminem for his fourth studio album "I Decided" (2017).  The song was written by Big Sean, Francis Nguyen-Tran, Mark Sebastian, Starrah, The Lovin' Spoonful, Eminem, WondaGurl and John Sebastian and was produced by WondaGurl, Big Sean, and FrancisGotHeat.  This song contains samples from "Summer in the City", performed by Quincy Jones featuring Valerie Simpson and was written by Mark Sebastian, Steve Boone and John Sebastian.  The song debuted at number 22 on the Billboard Hot 100.

John Sebastian (classical harmonica player): John Sebastian, born John Sebastian Pugliese (April 25, 1914 − August 18, 1980), was an American musician and composer known as a master of the classical chromatic harmonica.  He was the first harmonicist to adopt an all-classical repertoire and, along with Larry Adler and Tommy Reilly, established the harmonica as a serious instrument for classical music.  In addition to performing, Sebastian increased the range of classical music available for the harmonica by transcribing numerous existing classical works for the harmonica, composing works of his own, and commissioning or otherwise encouraging other composers to write for the instrument.

Supporting paragraphs: John Sebastian Woodstock
Answer: Woodstock