The Solstice does not affect the appearances of night time animals; they will still show up at their usual times.
Isabelle will spend the Solstice in the town square. If you talk to her, she will give you a pair of red Ladder Shades. As she is spending the day outside, you will be unable to access the Town Hall, pass ordinances, or start public works projects. Isabelle stands next to a face cut-out board with a Sunflower on the right and a Sun on the left. Unless another event is happening at the same time, for example a Gracie Fashion Check , or a Bug-Off or Fishing tournament , then Isabelle will stand out side the town hall, but the face cut out board will remain In the same position.