"More than 20 people were killed when a kamikaze bomber detonated his explosives at a gathering in Hasakeh on the eve of (Kurdish new year) Nowruz," Observatory director Rami Abdel Rahman told AFP.
Dozens more were wounded, he added.
There was no immediate claim of responsibility for the attack, but Abdel Rahman said the suicide bomber could have been a member of the Islamic State group (IS).
He said a second bomb exploded at another Nowruz celebration, wounding dozens of people.
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IS controls several parts of the province while fighters from the Kurdish People's Protection Units (YPG) hold the provincial capital, which is also called Hasakeh.
The YPG are sworn enemies of IS, which they expelled from the border town of Kobane in January after four months of fierce fighting with help from allied air strikes.
Nowruz, or Persian New Year, begins on March 21.