For city breaks, consider September to November or January to March. Yes, it might be a little colder in some European cities, but with better prices and fewer crowds, the compromise is more than worth it. For example, you can get two nights in Rome for around £100pp if you travel in October.
Article source: https://www.huffingtonpost.co.uk/entry/sunshine-saturday-best-travel-deals-hacks_uk_63b80e33e4b0ae9de1bd90a4