There's July 4th for the USA
July 14th for France.
A day off, fireworks, parties and something that brings a community together.
We have Bonfire Night - but I suspect that isn't all that unifying.
We have the different countries of the UK - which could also complicate if the wrong day is chosen.
It just seems nice to have a day for the country like some (quite a lot?) of other countries have.