By Apostrophes, Proper Grammar

Thank you for the reply.What about when an apostrophe is used for shortening the ing on a verb? Say for example: the man just kept on runnin'Is this proper grammar or just slang? If anyone knows what this is called, then please let me know.

I am working on an invitation and would like to use proper grammar. Cortnie and Josh have different birthdays, but are celebrating on the same day. How should I write "birthdays"?