Bonjour ! So your question is about "l'hiver" ou "en hiver", both are correct. "En hiver" means you are talking about the season itself (when the weather is cold etc). "L'hiver" has a *slight* difference, it includes a habit meaning like it's something you would do every year. But the difference is so tiny people actually use them interchangeably. I will add this though : be careful about the rest of your sentence. As you know, "je vais skier" can mean "I go skiing" but it also could mean "I'm going to ski" (le futur proche). To avoid any confusion, you may use "je pars skier" (and add the location because we usually include one when using "partir"). Happy skiing ❄