In the episode, iWill Date Freddie, Valery says she wants to do a webcast every Tuesday afternoon, and Freddie says that is the day they do iCarly. In iWant a World Record, they want to break the record for longest webcast for over 24 hours. If they do iCarly on a Tuesday and they were at school the day before when they brought in the book of world records, what ever happened to school? They would have to miss the whole school day on Wednesday.
However, it is possible that they did a special Friday or Saturday night webcast so they could break the world record.