When does danger from frost begin and end for Plano, Illinois, 60545?

Hi & welcome!

According to the zone map (accessible on this site, put your ZIP code in the box) here: https://garden.org/nga/zipzone...
you are in Z5b.
Clicking on the "View our Planting Calendar for your area" link leads to the information you seek: https://garden.org/apps/calend...
"On average, your frost-free growing season starts Apr 29 and ends Oct 9, totalling 163 days."

As soon as you can get a forecast through the end of April, you should know when you've seen the last of Jack Frost this spring.

Your probably in Zone 5 so that would be May 15 and October 15, hope that helps
To The Forum

Your good Tiffany what a difference a letter makes

TY for the kind comments. I lived in 5b in central OH for decades before moving to AL. There does seem to be a huge difference between the parts of Z5 that are very close to Z6 vs. the parts that are much closer to 4. The diff between A & B within a zone seems to be the most dramatic in Z's 5 & 7. Z4 has such a long period of cold, the start of arctic conditions and deeply frozen ground where it's rare to hear of instances of successful zone-cheating and creating a micro-climate of non-frozen ground, and Z8 is where the ground does not freeze, reliably, the start of where some tropical species are able to survive as herbaceous perennials, and some temperate plants start to fail.

Thank you, very good answers