It's a game set in Japan in August 1975 where you play as a little boy staying with his relatives in a small coastal town. I was going to try to sum it up myself, but my main exposure is watching a video about the first game. The Wikipedia article for the that game does a pretty good job summing it up (there are some additions to the second game like swimming, and it's a different family, but the basic gameplay is much the same):

Boku no Natsuyasumi is an open-ended simulation game self-described as a "nostalgic adventure", in which the player's actions determine how Boku spends the thirty-one in-game days of his summer vacation. Beyond the mandatory daily activities of morning radio calisthenics, breakfast, dinner, and bedtime, the game imposes no specific objectives or obligations of gameplay progression, and the player is free to spend their time as they see fit. Various activities can be undertaken by Boku, including but not limited to exploring the town and its surrounding environment, catching bugs, fishing, watering the Sorano family's morning glories, talking to the local townspeople, and flying a kite; the player can alternately choose to do nothing at all. The game's overworld is composed of a series of non-scrolling screens, and the in-game clock advances slightly whenever the character moves between screens. Each day ends with Boku recording the events of the day in his picture diary.

I just played my first day and, while you're a bit limited on that first day (you arrive later in the day and can't explore as much), the vibes are already immaculate.