A stitch has been left in my mouth for 2 and a half weeks. Is this dangerous ?

The remaining stitches will not cause problems. Just keep the sites clean as instructed by your surgeon. And when you see him/ her next, mention the stitches. It could be removed at that time. 

Since you have an appointment in one day, you should inform your surgeon about the stitch so he can remove it. To answer your question, for the most part, a retained suture will not cause damage or severe infection.  Just keep the area as clean as possible.  I would recommend not letting the suture remain there for more than another week in case you cannot see your surgeon for the current appointment.