UPDATED: The Legendary Week

[BlackSniper]

Just to avoid some confusion, the event will end around 11pm BST (extra legendary drop chance).
Following the end of the extra legendary drop chance will be the start of double XP.

Very rough estimates on time:
Legendary week end - 11pm BST/6pm EDT
Double XP start - 12am BST/7pm EDT.

At the beginning of next week (August 24-August 30), the NI Developer Team will start a new type of event.  All week long, old and rare legendary items will have significantly increased drop rate.

According his thread, extra loot will be the entire week and in the weekend double xp will be added as well. His countdows also say that the legendary event will be taking 2 days and 9 hours more.