If you have to sail during holidays and you don't have any choice – it's peak season, but plenty of people, ports are crowded and prices are extremely high.
If you can, you better sail in May–June or September–October. Ports are not crowded and water traffic is less intensive. In addition, prices are much lower. Moreover, wind conditions are the best in spring and autumn. In spring, days are longer. In autumn, the sea water is still warm.