It's an old episode where Eric takes Donna out for Valentine's Day and they end up getting drunk. The very beginning of the episode we are told it is Sunday February 13th 1977 i.e. the day before Valentine's Day. Eric says to Donna "I was hoping to take you out tomorrow night for Valentine's Day". This would mean that the next day would be Monday, right? Not according to 1970s Wisconsin, because Kelso later says to Hyde and Fez "Eric and Donna are going out Saturday night". This would either mean going back 24 hours in time where the date would February 12th or going forward a week when it would not be Valentine's Day.