Amazon Italy have listed Splatoon 2 for Nintendo Switch as releasing on August 15th. Whilst it could just be a placeholder date, it definitely matches the tentative release date of Q3 which is set between July and September.
Nintendo has also stated that the Nintendo Switch proper online experience will be launching around this time.
Splatoon 2 will have an online Testfire on March 25th and March 26th. You can find out more about that HERE.