LetItSnow! Yes, May 2020 was an exceptional month for the reason it had a potent cold spell and also a very warm spell, though I think 2012 is still on top for that exceptional heatwave in the final 10 days. The mid month northerly brought temps down to -3.5C here, which is quite astonishing for how late in the year it was. Keep in mind Summer was only 2/3 weeks away.
May frosts actually aren't as rare as I first thought, especially up north. Other May's where I've witnessed frosts in the last 10 years are 2015, 2018 (surprisingly), 2019 and 2021, though 2020 was by far the latest.