“wenn” and “wann” both translate to “when” in English, but they are used in different contexts and have distinct meanings.

TL;DR
Use wann when asking about a specific time or point in time.
Use wenn to introduce conditional clauses or describe recurring events or situations in the past.

wann

• Used to ask about a specific point in time.
• Typically used in direct or indirect questions.
• Directly translate to “when” in the context of asking about time.

e.g.
Direct question: Wann kommst du an? (When do you arrive?)
Indirect question: Kannst du mir sagen, wann du ankommst? (Can you tell me when you arrive?)

wenn

• Used to describe conditional situations or when talking about a recurring even or situation in the past.
• Translates to “if” or “when” in English, depending on the context.
• It introduces a condition or a hypothetical situation.

e.g.
Conditional: Wenn es regnet, beliben wir zu hause. (If it rains, we stay at home.)
Recurring event: Wenn ich nach Berlin fuhr, besuchte ich immer das Museum. (Whenever I went to Berlin, I always visited the museum.)