Nintendo makes an announcement of how long an even twill last, so you can probably check their site, or most pokemon sites, like this one, will also have information.
You can also go to: • Bulbapedia: http://bulbapedia.bulbagarden.net/wiki/Main_Page
~or~
• Pokemon website: Http://Pokemon.com
there's actually a Suicune, Raikou, and Entei event going on soon in January at Game Stop.
