Cecily (SESS-ih-lee) is the female variant of the Latin name Cecil, meaning "blind." This name can be found as early as 1246 with that spelling, and as early as 1210 as Cecilie. Going back further, Sisley is recorded in 1154. Cicely (SISS-eh-lee), however, is the later version of Cecilia (from Cecil, originally spelled Caecilia). Both of these sweet names can have the nickname Cece, although some might prefer the unusual nickname Celly. Or maybe you can borrow the Slavic name for Cecilia, Cilka.
Cicely ranked twice 1973 & 1974 but was only given to 16 girls in 2016, and Cicily to 5 girls. Cecily is much more popular - given to 194 girls in 2016, just below the top 1000 (but still far enough removed from it to classify as uncommon, perhaps even unusual).
Most notably there is King Richard III and Edward IV's mother Cecily Neville aka the "Rose of Raby." Cecily of York, the Viscountess Welles, was King Edward IV's daughter. There's also Saint Cecilia, who founded a church in the Trastevere part of Rome. She is the patron saint of musicians.
Other namesakes for Cecily include the creator of the Gossip Girl books Cecily von Ziegesar, actresses Cecily Strong, Cecily Polson and Cecily Adams, and painter Cecily Brown.
You might recall that Cecily Parsley is the cute Beatrix Potter rabbit, and Cecily Cardew is a character in The Importance of Being Earnest. The name Cecily has been used in other fiction, including Cassandra Clare's The Infernal Devices series.
For Cicely, real namesakes include actresses Cicely Tyson and Cicely Courtneidge, author Cicely Mary Barker (do a Google search and you're sure to recognize the work), and Cicely Mary Saunders - the nurse who founded the hospice movement. Don't forget English suffragette Cicely Hamilton, or maritime historian Cicely Fox Smith. In fiction, Lady Cicely Waynflete is a character in G. Bernard Shaw's Captain Brassbound's Conversion, and other Cicely's can be found in Sir Walter Scott's Kenilworth and the Dos Passos Manhattan Transfer.
Also, Sweet Cicely is a plant native to Europe. It is also known as garden myrrh and sweet chervil, and the leaves are medicinal and edible.