The main flowering of most lilac varieties, especially the common farmer's lilac (Syringa vulgaris), is in May. But there are stragglers who take a little more time.
The autumn lilac shows a re-flowering in autumn and forms buds from May to October. Inspect your lilac regularly, even during flowering, for signs of disease or pests such as leaf miners. If sub-zero temperatures are still announced in spring, a garden fleece is helpful.