I don't know when and where it rains on Sunday, but check the places where it rains the most for you
When I evolved my sliggoo into goodra, I usually went to Route 14 or Route 16 where it rained the most. If it's not raining, just catch that Magikarp you always wanted or something else to pass the time
Hopefully this answers your question
Hope I helped!(ish)
Source: Experience
