There are two theories as to whom Paul is writting - either to Christians in Northern Galatia or in Southern Galatia. If written to Northern Galatia, it would have been written during his second missionary journey (about A.D. 50). If written to Southern Galatia (this is the most widely accepted theory), it would have been written just before the Jerusalem Council, thus giving it a date of A.D. 48.

Source:

A Survey of the New Testament, Third Edition, Robert H. Gundry, 1994, A Survey of the New Testament, Third Edition, Robert H. Gundry, 1994, Zondervan, pages 345-346