Background odds

What's the chances of getting an event pokemon with a background? Like Go Fest Rayquaza, or Sarari Zone Eevee. Some say 1/5, some 2/5, or 1/3 I don't know the actual odds so I wanted to run some calculations myself. Hope I get some answers!
2 Replies
𖤐ʀɪᴏᴜ 𖤐
Taken directly from Reddit: “It's guaranteed the first time. It's 1 in 3-5 afterwards. However just raid like usual don't catch the Pokemon until it's shiny then catch. As for Kyogre and groudon they were special. You could only catch 1 set of both so if you willy nilly caught them without shiny hunting then basically you were locked out of the raids that gave the location card permanently for the event they were in”
Fubster
Fubster6mo ago
Why not just shiny hunt until it's the first time?