Aqui es una costumbre
Here is a custom.......apparently I am wrong it should have been 'Here its or it is a custom'
I want to know how en and una gets to its or it is?
By the way, we in Britain favour the word 'its' without the apostrophe.
Its needs an apostrophe if it's a contraction (as opposed to a possessive,) The subject is often assumed in Spanish.
I'd like to correct the statement that in Britain we favour 'its'!
It's = it is (as bjorseth says, a contraction of two words): It's late so I'm going home. Its = possessive: The dog lost its ball.
Most of the time, you don't need to use the subject as it is obvious from the verb itself and the context. The 3rd person singlular (and plural) require it more often however, as it could be el, ella or usted (or the plural version).