There does seem to be a lot of confusion around this, but according to the Season 1 blog post, new Seasonal Challenges that reward Tix will be added to the existing pool each week, and last until the season's end. Those will be how you complete the Seasonal Task for completing 120 goals.
The section from the blog post that XxBPMxX posted is missing the preceding sentence that states the bullet points are in reference to Tasks, not Challenges. Event tasks are not currently available, since the two seasonal events haven't started yet. Skate-O-Ween runs from Oct. 21 - Nov. 11 and 7-Ply Maple Harvest runs from Nov. 18 - Dec. 2. But to my understanding, Event Tasks don't reward Tix and will instead reward event currency for event specific cosmetics, though I could be wrong.
So TL;DR: You earn Tix from Seasonal Challenges, with new Seasonal Challenges added to the existing pool each week. As well as Daily/Weekly/Seasonal Tasks. Plus, if you own the premium skate.Pass, there is a free daily claim of 100 Tix from the in-game store.
https://www.ea.com/games/skate/skate/news/skate-season-1