When you say “all days similar to this one”, are you talking about all real days or all possible days? If it’s “all possible days”, then this seems like summing over the measures of all possible worlds compatible with both your experiences and the hypothesis, and dividing by the sum of the measures of all possible worlds compatible with your experiences. (Under this interpretation, jessicata’s response doesn’t make much sense; “similar to” means “observationally equivalent for observers with as much information as I have”, and doesn’t have a free variable.)
When you say “all days similar to this one”, are you talking about all real days or all possible days? If it’s “all possible days”, then this seems like summing over the measures of all possible worlds compatible with both your experiences and the hypothesis, and dividing by the sum of the measures of all possible worlds compatible with your experiences. (Under this interpretation, jessicata’s response doesn’t make much sense; “similar to” means “observationally equivalent for observers with as much information as I have”, and doesn’t have a free variable.)