Assuming you are talking about a real leather jacket and not a faux leather or vegan leather jacket, there are actually quite a few occasions where it is not appropriate to wear a leather jacket. Here are some examples:
1. Formal events: If the event you are attending is formal, such as a wedding or a job interview, then you should definitely leave the leather jacket at home. It will come across as too casual and could ruin your chances of impressing people.
2. Funerals: This is another occasion where it is best to dress more conservatively and leave the leather jacket behind. Wearing something like this to a funeral would likely be seen as disrespectful.
3. Hot weather: Leather jackets can be very warm, so wearing one in hot weather is probably not the best idea unless you want to sweat profusely! If it's cold out but not freezing, opt for a lighter weight coat instead.
4. When rain is in the forecast: Unless you have a waterproof leather jacket, it's best to avoid wearing one when rain is expected since they can easily get ruined if they get wet.