The other commentators are wrong about its usage. It is used as a name in Germany among girls. I have seen it plenty of times in the German Birth Announcements. It may be a newer name, but you certainly would not look insane if you used Sommer.
― Anonymous User 5/2/2012
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It does mean summer but it is not used as a name in Germany. If you named your child that over here, people would probably consider you insane. [noted -ed]