Translation:On Saturdays we go to the house of my parents.
Having the apostrophe after a plural is correct to show possession. You could say "your father's house" or "your parent's house" since your father is your parent. 'Parent' is singular. Your mother and father together are your parents. Referring to their house would be "your parents's house" [the s's is intentional] but this is incorrect in English as the pronunciation of the plural possession would be "parentses". So the final "s" is dropped. The correct way of writing it is "your parents' house".
This same rule is applied for any possessive noun ending in "s". For example, say "Porters" is the family name of some person then "Mr. Porters has a book" is the statement, so "that is Mr. Porters' book" would be correct, whereas "that is Mr. Porters's book" is not.