The phrase "the sun coming out" can symbolize emerging from dark times and returning to brighter days. It can also remind people of carefree summers, growing gardens, and wanting to return to "normal".
George Harrison of the Beatles wrote "Here Comes the Sun" in 1969 while staying at his friend Eric Clapton's house in response to the band's dark mood. Harrison recalled that he wrote the song when Apple was starting to feel like school, and that spring seemed like a long time coming after a long winter in England.