There’s a lot of unknowns.
Nobody knows if the Copenhagen Jazz Festival will be able to be held. Nobody knows which travel restrictions will/won’t be in place in July 2021.
I can’t seem to find any information about Copenhagen Jazz Festival 2021 on the official website www.jazz.dk. Be sure to check up on that, if the Jazz Festival is a main motivator for this trip.
Currently, all travellers arriving to Denmark are encouraged to self isolate for 10 days. It’s not the law, and you can’t be punished if you break the isolation. Danes are very trusting and rule following, so most people would actually self isolate for 10 days even though it’s “just” an encouragement. However, a few people haven’t stopped all unnecessary travel activity, which have started political discussions about introducing mandatory isolation when entering the country. All of this could change, and my layman’s assessment of the situation is that this will change by July 2021.
Current projections predict that all Danes will be vaccinated by the end of June 2020 (all who want it, of course). That will depend on the delivery of the vaccines, and there’s a lot of news about delays these days.
When it’s something that you want to do, and something you have the budget for: Go for it!
As others have said, don’t book anything non-refundable. With current rules and restrictions, the event and travel activity we’re discussing here is 100% prohibited.
The situation will certainly be different in July, we just don’t know how different it’s going to be yet.