Here is some most typical places for April training camp:
Ahvenanmaa (Åland)
http://www.idrott.aland.fi/orientering/
One of the best places on earth for orienteering, The Bare Rock Paradise. You need to go by boat. Possibly no practice courses.
Parainen
http://www.piforienteering.fi/news/show/66
Very popular place for April training camp. You'll find practice courses there, lot of very nice maps.
Paimio
http://www.paimionrasti.fi/
Not so snow proof, go there only if you know there is no too much snow.
Turku:
http://www.tus.fi/index.php?language=english
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There is no much information yet, but guess soon they will have adds there.