Early Spring Frost Tolerance
Below are links to current data on early spring frost tolerances by bud developmental stage.
One link is from the UMass Cranberry Station (updated 2015). They've done an excellent job of providing high quality photos to reference.
The second link presents the tolerances by bud stage as determined in David Bryla's frost study in Oregon. As he noted in his presentation at the 2016 Oregon Cranberry School, his findings were similar to those found in Massachussetts.