The internet is so fucked. I just looked up how/when to tap birch trees for sap, and the article I found talked about how tapping birches is where maple syrup comes from.

This AI slop is like microplastic. It's polluting everything.

@Shanmonster I search a lot about nature/science, so my tip is to start with Wikipedia and build bookmarks with real human platforms where you know, it's no slop.
Probably you won't find exact dates because it depends on the region and the weather.
Finding humans who do it in your region, can help.
Short version: Yes, internet search is fucked. The good internet is still there but becomes more and more invisible.
@Shanmonster Just a PS because I live in a region where they collect it: The best time is after the winter frosts when the sap is going up by the first warmer days. And they stop collecting it when the buds open to a small green: then the tree neeeds the sap for itself.
That's a purely human information by farmers.
@NatureMC I figured it would be when the sap first begins to run. My dad used to run a maple line in the 70s. I was just looking it up because I'm writing a story set in the spring, and I wanted to doublecheck that that was too late to tap birches.
@Shanmonster Ah! Birches have a different sap/weather behaviour than maples. So the link that @natematias has provided here, could be helpful for you: https://blogs.cornell.edu/cornellmaple/about-birch-syrup/
About Birch Syrup | Cornell Maple Program