MMC talvisota, aka winterwar near tampere, dont know if its biggest or anything but last years theres been some 80 cars and atv´s on site and date for this year is 2nd of march
(this is from nordic trial but still, same place, raisio)
winter with all that snow is mostly limiting all could we say "your type of offroading", its just wading trought the snow for everybody, and what comes to your date than mentioned above , 8th of may... tmk trial is just nearly after, 15th may in raisio, near turku.