After every 1month this summoning event arrives. I just play every day redeem from events with afk ,base, forest points.. also from special training completing 30 stars it gives 10/15 scrolls....promotion it refreshes every 1hr you also get some scrolls from there.
Once you get any SP ninja It will be added in special summons....You might notice I don't have Sasuke (Rinengan) and Naruto (Six path) in my summon because I didn't get them once.