L’ or En

Question

How do know when to use these before the season?

Accepted Answer

As the lesson says, you're free to choose. Both are correct although there seems to be a subtle difference in meaning:
En hiver refers to the season in a more general sense (e.g., il neige en hiver)
L'hiver talks more of a specific winter season (l'hiver on fait du ski).
Now, I may be imagining this subtle difference. Maybe one of our kwiziq natives can comment on that?

Answer

Bonjour Gary,
Chris is correct! Remember it is a subtle difference though:
En hiver -> it implies the duration of the season
il neige en hiver 
-> during the whole season
L'hiver -> it implies that specific season
L'hiver on fait du ski
 -> at that specific time of the year
 
I hope this is helpful.
Bonne journée !