I’ve been doing natural dyeing demonstrations for many years. Although nearly any green plant will produce some shade of yellow, I was never satisfied with the results. About 5 years ago, in a search for a clear, golden yellow dye to make some muffatees with my homespun, I remembered the orangey sap that oozes from celandine when it’s pulled up. This plant grows wild in several areas of my yard, so I collected some and gave it a try as a dye. The results were spectacular: a soft, true, buttery yellow.
And it was easy! I simmered the plant, roots, leaves and all, in about 1 gallon of water for about 1/2 hour, then removed the plant material. The yarn was mordanted in alum and cream of tartar, and submerged in the celandine bath. I simmered (not boiled) the yarn for another 1/2 hour, and was so happy with the results.
Celandine in New England begins to blossom in late May or June. The plants are perennial. Oh, by the way, the juice of celandine is supposed to be a great remedy for piles (hemorrhoids, but as I haven’t tried it myself, I’m not making any promises!
You're History! 
I have tons of this stuff in my yard. I’m not a dyer, but I did take a wonderful natural dye class at Fiber College in Maine, just for the fun of it. Good yellows are hard to find. Perhaps I’ll play with some if the sun ever comes out!
M, let me know how it comes out. Good luck!