Oops, it doesn't look like 'best mates day' is a national day..

Other days

unicorn

Unicorn Day
blanket fort

Blanket Fort Day
ruby

Ruby Day
kale

Kale Day
hoodie

Hoodie Day
lash

Lash Day
pick me

Pick Me Day
boyfriend

Boyfriend Day
ask out your crush

Ask Out Your Crush Day
tulip

Tulip Day