When to start tapering??

I normally taper the week prior, but would also consider how long you have been training for it.  Give yourself a solid week to recover from any intense workouts, you will be amazed at how good it will feel to go into the race fully recovered. 

Spend a little time the week prior mixing up activities at very easy levels, if you have a 5k, go for a nice 1-2 mile run at a very easy pace early in the week, follow it up with some easy biking, and just spend a bit of time in the pool to keep yourself limber.  I have found that doing each activity the day before for 10 minutes very easy helps me feel more prepared.