Because of its etymology (elision of a masculine singular phrase), סֻכּוֹת \ סוכות, though feminine plural in form, is generally treated as masculine singular.
Originally חג הסוכות \ חַג הַסֻכּוֹת(khág hasukót, literally “Festival of Booths”), from חַג(khág) + סוכה \ סֻכָּה(suká, “booth”), after the huts lived in during the holiday, or after the ancient huts the modern ones commemorate.