Oops, it doesn't look like 'stop dan howell day' is a national day..

Other days

slime

Slime Day
tulip

Tulip Day
unicorn

Unicorn Day
artists

Artists Day
ruby

Ruby Day
boba

Boba Day
my little pony

My Little Pony Day
lost sock memorial

Lost Sock Memorial Day
nail polish

Nail Polish Day
kale

Kale Day