The Roman festival of #Regifugium normally fell the day after #Terminalia, 11 days after the ides of Februarius and 5 days before the kalends of Martius. But in a leap year, Mercedonius would sometimes fall the day after Regifugium; other times, it would fall the day after Terminalia, in which case Regifugium wouldn't fall until 27 days later, 5 days before the ides of Martius.

The Romans were the *worst* at timekeeping.