When you are talking about date, 号 and 日 are interchangable.
For instance, 4月15日 and 4月15号 are exactly the same, both meaning 15th April.
Given the context, they can also be used alone without referring to certain month, e.g. 15日 and 15号 both meaning the 15th day of the mentioned month.
Of course they have different meanings when you are NOT talking about dates.
From the translation you mentioned from Google, 号 also means 'Number'.
The 'Number' here refers to ordering. For example Patient Number Five is translated as 五号病人.
In addition, 日 has the meaning of Day in the sense of a particular occurance or purpose, e.g. 世界(World)环境(Environment)日(Day).
Note that when you are counting days without referring to certain month, you cannot use 号.
For example, Day Six has to be translated as 第六天/第六日 instead of 六号.