Il va de soi is a French impersonal expression meaning “it goes without saying”.
Il va de soi
It goes without saying, needless to say

Example sentences
Il va de soi que la famille passera Noël à Paris.
It goes without saying that the family will spend Christmas in Paris.
This sentence uses the verb assister, which can be tricky. Assister + person means “to help” and assister à + event means “to attend”.
Il va de soi que nous assisterons à la fête d’anniversaire.
It goes without saying that we’ll attend the birthday party.
Expression origin
The origin of this expression is unkown. Wiktionary gives the the following French definition: La conséquence logique de ce qui a été dit précédemment ; être facilement déductible (The consequence of what was said before; To be easily deductible).