Thousands of Monarch butterflies overwinter in Pacific Grove, California, at the Monarch Grove Sanctuary mainly during November through February. A couple years ago we visited in February and saw so many of them in the eucalyptus trees. I’ll have to find a photo to show you, but there are some good ones on the website linked below. We did see a few fluttering around last weekend, as well, even though it was already April. It is always a special visit.
The Pacific Grove Museum of Natural History runs the museum, the Point Pinos Lighthouse, and the Monarch Sanctuary.