We had a week of mid 60's, I waited. For the second day of 66° and sunny with no wind. I was quick and did my check and got out. My bees have been bringing in pollen all winter so I made the executive decision to open 'er up and do a check considering I did have a horrible case of varroa in the fall. I don't regret my decision, I do appreciate all the different advice tho!About 8 years ago we had an unseasonably warm January and several swarms were found in January and March in North London.
That combination of mild winter and very early spring is rare, but it does emphasise the need for beekeepers to be in sync with the seasons and avoid going by the book.