Question

why do we celebrate a national holiday

Answer 1

It depends.For eg:- We celebrate Independence Day as we got Independence on that day and to pay respect to the people who have struggled for it.
We celebrate Gandhi Jayanthi to pay respect our own Gandhiji and his continuous struggle for freedom.
Hope it helps and pls mark as the best answer. :-)

Answer 2

to show that day is important happened like independence day etc