There are two draw events where the chance of getting an SSR from a draw is increased to 6%. The flash gala and premium gala.
The premium gala contains the "Grand" SSR versions of the main story characters, along with that year's Zodiac characters. The flash gala contains non-main and non-story characters.
For a new player, Summer Zooey and Lucio are core characters for dark and light. If you're going to do your first spark, it's not a bad idea to do so when one of them is on rate up so you have a ~50% chance of getting one from the 300 draws.
You can see a list of the limited characters available in each here: