The best time to visit Slovakia is between December and February for skiing.  The best place to ski is High Tatras mountains (Tatranska lomnica).  You can fly straight to Poprad City, which is closer to High Tatras than the capital city of Slovakia,  Bratislava city.  A good site about Slovakian mountains is and for more info  There are also reasons to visit Slovakia in the summer.  There are a lot of places with open swiming pools and many historical castles.  For exampe, example in Trencin there is the castle on the top of the hill .  In Slovakia are many villages where you can see traditional lifestyles and many traditions.

A good time to visit is after July, gettting into August and September. The weather is still warm enough to permit wandering around without a lot of outerwear.  Most of the tourists are gone, and the new wines are available at festivals and wineries. Usually by the time August and September come, hotels are readily available, the harvest is on and the nights tend to be cooler. Some tourist stops are open but not congested (Slovak Bethlehem, Cicmany, Zilina, Trencin, Kosice, etc,)  Stay until the All Saints-All Souls traditions can be experienced. Travel in Slovakia is so much more relaxed than many other places, and it is never too crowded at cultural events, castles, concerts, etc,