in Thoreau’s Journal:

When the wind is still cool elsewhere, I glance up some warm southern slope, sunny and still, where the thinly scattered blades of green grass, lately sprung, already perchance begin to wave, and I am suddenly advertised that a new season has arrived.

This is the beginning of that season which, methinks, culminates with the buttercup and wild pink and Viola pedata.  It begins when the first toad is heard.