The same day; on that day (Revised Version). Although day is sometimes used in a metaphorical sense, so as to include what is, in fact, a long period of time, yet we are not justified in assigning this sense to it unless the context clearly requires us to do so.
This is not the case here, so that we must assume that a literal day is intended. But which day? Naturally, the day that has just before been mentioned, either in the original source from which our narrative is taken or in the narrative as it now stands.
Since, however, Matthew 12:46-50 and our Matthew 12:1-23 appear to have been already connected in the framework, these supposed alternatives really represent the same thing, the phrase probably referring to the day on which our Lord's mother and brethren sought to speak to him (Matthew 12:46).
Went Jesus out of the house. Where he had been when his mother came (Matthew 12:46, note), and presumably the one to which he returned in Matthew 12:36. Possibly it was St. Peter's house at Capernaum (Matthew 8:14).
And sat (Matthew 5:1, note). By the seaside. Until the crowds compelled him to enter the boat.