Ponyo is just like infancy. Cute, genuine, fantastic, carefree, heartwarming, when everything is blossoming with colors, is cheerful, when everyone is friendly and well-spirited, even if their actions seem initially a little shady. Nothing bad ever happens, everything ends well, there's absolutely nothing to worry about at any moment.
A movie without conflict might seem, initially, pretty shallow, but if anything Ponyo ends up being more of a slice of life type of thing than a movie as we know of it. So much so the attention to details is painstaking - the characters' movements are genuine, realistic and offer extreme continuity. If Ponyo is holding a bucket, in order to grab something on her backpack, it will be shown Ponyo dropping the bucket, picking her object and then grabbing the bucket back. This attention to minuteness makes Ponyo more interesting.
Completely devoid of any pretension, all this movie wants to do is be heartwarming and visually outstanding. And it succeeds.
Imo it fits perfectly on a melancholic day (to cheer up) or for a vividly bright day (to maximize the potential for happiness).
Either way, PONYO IS PURE JOY!