The number of events taking place is considerably higher than the number of events that gave away Darkrai last summer, with multiple events taking place across the UK on every day between the 4th and 18th of April, except for the 6th, 7th and 14th of April. The usual event activities will be taking place at those locations as well.
The Shaymin events will also be held in Spain starting from April 15th at GAME stores, and it's likely that France, Belgium, Italy and the Netherlands will be having their events start at the same time as the Spain events. Germany's events will be being held in ten McDonald's restaurants across the country between April 25th and May 17th.
More information on the event dates/locations in the UK can be found here: http://www.game.co.uk/pokemonevent/
...and Nintendo's Official Shaymin site can be found here: http://ms.nintendo-europe.com/shaymin/