@Larissa, lilac bush clones are used across the US to provide information on seasons and indicate climate change. I wonder where in this country the lilacs are blooming this late ... Alaska, Wyoming, Colorado but I bet it's above Denver's altitude.
Near the Idaho and Washington border is the "lilac city," Spokane. The Lilac Festival is early May and there are usually flowers. Sometimes they haven't yet bloomed and some years (like 2016), they are past their prime!
We have a well-known poem about lilacs blooming at the time of the death of a US president. He was killed in mid-April, at the time lilacs were blooming in Washington D.C., evidently.
Steve