Summer is the English word for the warm season — from the Old English sumor, with cognates across all Germanic languages.
Today it sits in the U.S.
Summer 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 Summer reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.