Last fall I visited the Berkshire in NY. I was talking to a person that owns a maple syrup farm... When they start pulling the sap they have a short window to work with... they pull sap from the same tree but when the window closes they have 4 grades or grading of syrup... Fancy, Grade A Medium Amber, Grade A Dark Amber, and grade B. Fancy grade is the best and Grade B is the type for making candy or the type on ice cream.. So the early sap is when they get their best grade and as the weeks advance the poorer grades are pulled and all grades are used for different purposes as far as the industry goes.. So when you buy syrup in New England make sure you check what grade it is so your getting the most for your money.. Don't spend money thinking it's the fancy grade and it's the last Grade B. Not that it won't taste good on ice cream or for making candy.
what bruin said. Need thawing days and freezing nights. Keep in mind that the sun warming the bark can cause sap to flow even when it is only 32 degrees.
You can make syrup from the sap of any maple tree, though the sugar maples are best. You can also tap sycamore, hickory and black walnut. Black walnut sap makes a yellow buttery sweet tasting syrup. Some folks tap birches, but their sap doesn't run until much later in early spring. I met old timers who say Hickory syrup tastes the best of all.