I think not bothering with months at all would be
better than having a 13 month year.
The phases of the moon are 28 days long. So a 13 month calendar also lines up with the lunar cycle. The more you think about it, you wonder how they ever came to the conclusion to use 12 months instead of 13.
(http://en.wikipedia.org/wiki/Moon and see the synodic period (quoted to a tenth of a second even though full moon to full moon can vary by about ¼ day either way).)