I ride outdoors all year long, but only road bike (i.e. take long rides out on hilly rural roads) in certain conditions...so I'll play
For me, it has more to do with the road conditions than any particular date or temperature:
1) Most snow melted from roads, a chance of black ice: Road cycling on gravel bike with studs.
2) Snow and ice melted, roads still covered with salt/sand from winter: Road cycling on gravel bike with gravel tires.
3) Snow and ice melted, spring rains have washed away winter salt/sand: Actual road cycling on a road bike!
Here in western Wisconsin, this means that my "road" season usually begins in February or March (with numerous interruptions, as it will snow through April), and the princess road bike usually does not hit the streets until early May
