To reduce the number of days going to the hospital
To reduce the number of visits to the hospital
この場合の 日を減らす は to reduce the number of days と言えますが
後者の病院への訪問を減らす で to reduce the number of visits to the hospital の
方が表現しやすいかもしれません。
「病気の治療で毎日通院していますが行く日を減らすか考え中です」
"I go to hospital everyday for my medical treatment but I'm thinking of reducing the number of visits"
To cut down on the number of trips I make to the hospital
To not go to the hospital as often
「病院に行く日を減らす」は
to cut down on the number if trips I make to the hospital
to not got to the hospital as often
のように言えます。
例：
I have been going to the hospital for treatment everyday, but I'm thinking of cutting down on the number of trips I have to make.
「治療のため毎日病院に通っていますが、病院に行く日を減らそうと思っています。」
but以下を次のように言うこともできます。
... but I'm thinking of not going to the hospital as often from now on.
「今後、病院に行く日を減らそうと思っている」
ご参考まで！
日本 
カナダ 
