Oops, it doesn't look like 'abdominal stretch day' is a national day..

Other days

comfy

Comfy Day
bonang

Bonang Day
slime

Slime Day
gum

Gum Day
boba

Boba Day
nae nae

Nae Nae Day
bow tie

Bow Tie Day
reading

Reading Day
ask out your crush

Ask Out Your Crush Day
youth

Youth Day