The Blizzard of Olaf was a severe weather event mentioned in "Animal House". It was bad enough to get named.
The Blizzard of Olaf occurred before the events of "Animal House", within the lifetime of the adult villagers and possibly before the events of How to Train Your Dragon. According to Bucket and his increasingly tight bucket, another bad storm is approaching Berk. Stoick is skeptical, but Mulch reminds him that the Blizzard of Olaf was predicted by Bucket's bucket.
Later, when explaining the accuracy of Bucket's predictions, Mulch says it took a week to dig Mildew out of the ice left from the storm and the rest of their lives to wonder why they did it.