Halloween 1980: "17-year-old Rudy Farmer pulled out a pistol and shot a fellow student in the art room at Hueytown High School. That student was wounded. Farmer fatally shot himself."
Halloween 1984: An unknown individual kills 6 (and two dogs and two chickens) and injured 1 in Chrissiesmeer, South Africa South Africa
Halloween 2007: Xing Guoping kills six in Sanyo District. He is arrested and executed.
Halloween 2008: "Eleven fifth-grade students were taken hostage in a classroom at Stockton Springs Elementary School, in Stockton Springs, Maine. The hostage situation ended within 30 minutes when the gunman surrendered to a state trooper. The hostage taker, 55-year-old Randall Hofland, was wanted by authorities for pointing a handgun at an officer during a traffic stop and then fleeing."
Halloween 2012: "At a Halloween party on the University of Southern California campus, an argument escalated and a man shot Geno Hall seven times with a handgun, critically wounding him. He shot and wounded three other people, who were not students of USC. Brandon Spencer, 20, was arrested for the shooting. He is charged with attempted murder."
Halloween 2017: Sayfullo Habibullaevic Saipov kills 8 and injures 11 with a truck in New York.
Valentines Day 1889: Jim Jumper shoots at least 6 and injures 1 at a Seminole Indian camp.
Valentine's day 1920: "At the Durant Normal School, teacher Albert McFarland, was shot and seriously wounded by one of his pupils."
Valentines Day 2001: Khalil Abu Olbeh kills 8 and injured 26 in a vehicular attack in Azor, Israel.
Valentine's day 2005 "A 17-year-old boy stabbed two teachers and a school nutritionist with a kitchen knife at his former elementary school, killing one of the teachers."
Valentine's day 2008: Steven Phillip Kazmierczak kills 5 and injures 21 at Northern Illinois University.
Valentines Day 2015: Wu Shouguo kills seven members of his family with a blunt object then commits suicide with poison.
Valentine's day 2018: Stoneman Douglas High School shooting
Christmas 1887: Nathan Reid kills 7 in a familicide before committing suicide.
Christmas 1929: Charles Davis Lawson kills 7 in a familicide before commiting suicide.
Christmas 1955: Tawar kills 4 and injured 7 in Surabaya, Indonesia.
Christmas Eve 1973: Lindolfo Garcia de Vasconcelos kills in in a familicide with a melee weapon and arson.
Christmas Eve 1974: "6 students were killed and 10 others wounded, among them several foreign students, when 17-year-old Branimir Donchev began shooting with a pistol in a university hostel in Sofia. Donchev was subdued and arrested by police."
Christmas 1983: Zhang Zexiao kills 6 members of the Wei family.
Christmas Eve 1996: Heidrun-Erika Jung kills 2 and injures 13 in a grenade attack in Frankfurt, Germany.
Christmas Eve 2005: Yan Xueguo kills 4 and wounds 27 with poison in Qixing Village. He is arrested and executed.
Christmas Eve 2008: Bruce Jeffrey Pardo kills 9 and injures 2 in a familicide.
December 2011: Aziz Yazdanpanah kills 6 in a familicide before committing suicide.
Christmas 2018: Qiu Rihui hijacks a bus and drives it into a crowd, killing 8 and wounding 25.
Christmas 2020: Anthony Quinn Warner commits a suicide bombing in Nashville in an RV. He is careful to hurt no one by announcing the bomb's presence.
4th of July 1926: Gangadharan kills 2 and wounds 13 on the steamboat Teesta.
4th of July 1996: Mohammed Nawaz kills 14 members of his family in Faisalabad, Pakistan.
4th of July 2010: Dong Chuan-Sheng kills 23 and injured 19 in an arson attack in Wuxi.
April Fools Day 1996: Wang Xiangjun kills 7 and wounds 5 at two schools in Meitian with a kitchen knife.
Edit: Nathaniel Berhow committed his shooting on his 16th birthday. Not quite a holiday, but it certainly had the same signifiance to him (I personally take that date as a statement of self resentment or dissatisfaction.)