July and August are the busiest months, when the alignments attract their largest crowds and the beach fills with French families on summer holiday. For the megaliths, visiting outside these peak months is strongly recommended — the scale and atmosphere of the stones read very differently when you are not surrounded by hundreds of other people.
May, June, and September offer good compromises: warm enough for the beach, far less crowded, and with guided access to restricted sections of the alignments available. In winter, the stones are at their most elemental — the empty landscape and low light can be remarkable — but guided access is limited and much of the beach resort infrastructure is closed.