Sunday is from the Old English sunnan-dæg (day of the sun) — a modern American word-name. Sunday Rose Kidman Urban (born 2008) — daughter of Nicole Kidman and country singer Keith Urban; her name was widely covered in 2008 celebrity media; sparked the broader day-of-the-week naming trend in 2010s American culture; her sister Faith Margaret was born via surrogate in 2010. Sunday Times — major UK newspaper (founded 1821).
Sunday does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Sunday reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.