"Wenn" can mean "when" or "if". "Wann" can only mean "when." "When will you go?" requires "Wann," not "Wenn."
The following is from the WordReference forum (https://forum.wordreference.com/threads/wenn-wann.999943/#post-5236170: ) :
"Wenn means if or when in the sense of a logical consequence or in the sense of a causal relationship. It is the fact that a causal succession implies a temporal one which gives wenn a certain temporal connotation but the meaning of an if..then relationship is always the primary one."
Why would 'Yes, apart from when it rains' not be acceptable. It is how I automatically saw it