I had a very early Emperor in the garden last week. The fieldguides say flight period is April-May, but I usually put the lure out in late March and catch an outlier, but I saw 2-3 in the garden on the 17th. I will put the lure out earlier next year, as they're presumably emerging earlier these days.

https://www.sciencebase.com/science-blog/moth-of-the-moment-the-royal-succession.html