Random Riddle: Winter Is Dead

Can you figure out what A.A. Milne was referring to in this verse from his “When We Were Very Young” book?

She wore her yellow sun-bonnet,
She wore her greenest gown;
She turned to the south wind
And curtsied up and down.
She turned to the sunlight
And shook her yellow head,
And whispered to her neighbor:
“Winter is dead.”
 

Random Riddle: Winter Is Dead