Event

apparently between the 2 summer events.

From what I've heard it's just one big pool of servants, so there's no narrowing down the possibilities. According to this, you are guaranteed one each 3/4/5*

https://fategrandorder.fandom.com/wiki/Fate/Grand_Order_Fes._2017_%EF%BD%9E2nd_Anniversary%EF%BD%9E_Lucky_Bag_Summoning_Campaign

The next GSSR should be around 25 June 2019 with only 1 banner with all SSRs available until June in the summoning pool.