As with most Greek islands, the best months are May-June and September, just before or after the midsummer crowds. The sea is warm enough for swimming from May through October, and direct flights to Preveza tend to operate during this same period. July and August are busy, of course, but a few merciful degrees cooler than (for example) Crete, thanks to afternoon winds and occasional rain showers.
If you're here in August, check out some of the summer festivals, most notably the Folklore Festival (Liff) at the end of the month, and on the first Sunday in August, the Greek wedding bonanza in the inland village of Karya. Other festivals include the 7-July remembrance of Agia Kiriaki, the patron saint of Vlicho, opposite Nidri (evening mass and through-the-night music with wailing clarinets and circle dancing); and a similar event on 27th May at Agion Pateron convent above Nikiani.
Lefkas is linked by bridge to the mainland at Preveza. Preveza airport has domestic and international flights; it's 30 minutes' drive from northern Lefkada (see our car rental recommendations). You can also arrive by ferry from Kefalonia, Ithaca or Meganisi, and seasonally from Astakos on the mainland.
For an overview of airlines flying to Preveza, see our Greece guide.