Hi Broc,
If we do a search on Google, we can find something other than something related to liquids...
Thanks anyway!
When I do a search on Google, I find one result in English, and that's someone saying they are having a 'runny day' because they have a cold and their nose is running. So that's related to liquids, and it's not a standard expression.
So if you can tell us where you saw or heard the usage of 'runny day', maybe we can help more. Otherwise, as Copacetic and I have said, it's not an expression we use.