Chicago days under -15°F since 2015:
Jan 18, 2016 -21°F Coldest day of that winter
Dec 19, 2016 -21°F Early-season Arctic outbreak
Dec 27, 2017 -19°F Part of a prolonged late-December cold wave
Jan 2, 2018 -23°F Deep freeze to start the year
Jan 30, 2019 -30°F Coldest Chicago temp since 1985; “Polar Vortex” event
Feb 14, 2020 -18°F Valentine’s Day Arctic blast
Feb 7, 2021 -21°F Mid-winter cold snap
Dec 23, 2022 -23°F Pre-Christmas Arctic front
Feb 3, 2023 -17°F Last occurrence to date
So basically every year.
edit: downvoted for noticing that it is cold.