Spring fever. A feeling of restlessness, excitement, or laziness brought on by the coming of spring.
The trees in the parks are blossoming (see above) there's a special energy in the air and you feel strangely optimistic. It's springtime!
The term "spring fever" comes from a poem by Mark Twain, author of one of the great American novels, The Adventures of Huckleberry Finn.
Twain wrote in the 19th century, but the feeling he describes is timeless:
“It’s spring fever. That is what the name of it is. And when you’ve got it, you want – oh, you don’t quite know what it is you do want, but it just fairly makes your heart ache, you want it so!” :-)
Restless. Characterised by inability to remain at rest
Blossom. The state of flowering
Timeless. Restricted to no particular time