Robert J. Harris 
The Day the World Went Loki [EPUB ebook] 

Lewis and Greg have accidentally turned their mum into a troll, and the town of St Andrews into a fantasy world overrun by yetis, elves, dragons and valkyries — few of them friendly. When the brothers recited an ancient rhyme to conjure up an extra day of the week, they had no idea it would work — or that the day would belong to the banished Norse god, Loki, master of mischief, monsters and mayhem. As the day wears on, time seems to be lagging behind. Will this day of madness never end? Can two ordinary boys outwit an ancient lord and his monstrous minions?