I like Sunday, but as you see there are some legitimate concerns with it. I love the suggestions of Susannah & Summer! Some other suggestions to get the name Sunny......
Sundara- technically a male name but I think it would work well on a little girl
Sana- cute and also means brilliance, radiance, splendour
Sonary- This name has it's originates from the Thai language. Roughly translated, it means a beautiful girl (according to baby name wizard...)
Sunday is quite Dragnet to me... sounds like Joe Friday... I find it to be a guilty pleasure name, or great as a mn. That's mostly because, like Sunshine, it's a great name for a little kid, but not such a great name for a career... Imagine being a prosecutor named Sunday Jones... eek! It just wouldn't age well... My mom's nn was Sunny, and her name had nothing to do with the Sun (and didn't have Sun- in it), so choose a professional name and nickname her Sunny.
I like it, and it isn't cruel but it doesn't work. I have a test: try the name in situations. Say the ln is Johnson. Can you picture a...
Sunday Johnson, teacher
Sunday Johnson, attorney at law
Sunday Johnson, janitor
Sunday Johnson, PhD
Sunday Johnson, professor of physics
Sunday Johnson, first woman on Mars
I can't see some of these, so the name would just be a GP. I love Summer, but it doesn't past my test. Sunny for Susannah is best, IMO
Here's a link for a thread about names Sunny can be a nn for. I like the idea of Soleil.
I don't think it's too cruel. Nicole Kidman has a daughter named Sunday. Perhaps she has made it a little more accessible. The only thing though is people will probably wonder if Sunday has a special meaning to you? Like was she born a sunday? Are you religious? etc etc.
Another possible idea is to choose a name that means Sun...and just call her sunny as a nickname.
Some examples of names that mean Sun...
-Soleil (literally means Sun in French)
-Apollonia (feminine version of Apollo - The Sun God)
I had the same southern vibe from Susanna, until I realized it also has a long history in Britain as well. Shakespeare's daughter was even named Susanna, that really gives it a more intellectual edge for me! I find the spelling Susannah with an h seems southern still, but Susanna makes me think of Shakespeare and Susanna Clarke, a fantastic modern British author (she wrote Jonathan Strange and Mr Norrel), I think the historic and literary connection really make the name appealing!