This is my first year playing league so I'm not sure how old seasonal skins are treated. Will we have a chance to buy the older ones or are there only new ones being released every year? I really want Bewitching Nidalee xD
Yes. Every Harrowing, they release the legacy skins for a short time along with the new ones. Exact date isn't always clear/specified though.
Every season, holiday based skins get put back on sale to celebrate the holiday.
Example: all christmas themed skins were in shop during snowdown last year even though some were released in season 1.
