UPDATE 3pm UK: This season's event, the summer Sigrblot Festival, will run from 29th July to 19th August, Ubisoft has now announced.
Valhalla's The Siege of Paris expansion will land right in the middle of all that, on 12th August.
Preparations for the above will come as part of tomorrow's 1.3.0 patch, alongside a handful of new skills. These include Light Fingered (auto pickup nearby loot), Thrill of War (gain adrenaline while in conflict), Heidrun Slam (press R2 while sprinting to knock enemies back), Idunn's Heart (passively regenerate recent health loss after a short delay), Survival Instinct (when at less than a third of health, hold -> for partial healing in exchange for adrenaline) and Wolf Warrior (damage increases the lower your health).