That depends on the type of contract you have with your current supplier.
Do you have a fixed contract?
It is usually advisable to wait until that contract has ended. Many energy suppliers will charge you a penalty if you leave early. However, sometimes our offer is so good that it is worth paying that penalty.
If you have an “open-ended” contract, you can leave whenever you want (often with a 30-day notice period).
If you leave right after the winter, be aware that your previous supplier will send you a high settlement bill because they have financed the entire winter. On the other hand, if you switch after the summer, you will often receive a refund, since during that period you have paid more than what it actually cost.