You already know that I am a fan of phenology by my reliance on the oak leaves as a last frost date.
For years, my recollection (backed up by 20 years of written gardening journals) is that our trees are pretty much in full leaf by the first week of May.
Additionally, the dogwood trees bloom for Mother’s Day, along with the lilacs. Other gardeners that I have spoken to confirm these “recollections.”
So is the problem this year the date of Mother’s Day, which is falling in the second week of May? Because while I do have dogwoods (albeit not quite as lush this year as in other years), the lilacs are barely open.
Perhaps because of the late date, they decided to split the difference!
Gardendaze 
Phenology really really does make more sense than minding a calendar that plants are not minding. I mean, we are working with plants, not the accrual of interest.