Sorry, but it is not in Nakorn Sawan. It is actually much closer than that to Bangkok. It is in the district of Nakorn Chai Sri in Nakorn Pathom province. It is about 1.5 hours driving from Bangkok. The date changes every year because it is based on the lunar calendar, but normally it is scheduled for a Saturday. Here is what I know about this year:
Wat Bang Phra Wai Kroo Tattoo Festival
Saturday, 27 February, 2010
5am (starting time) – 9am (main presentation from the head monk)
Temple Address & Phone Number:
Phra Udomprachanat (Luang Por Phern)
Wat Bang Phra, Tambol Ban Gaew Fa
Amphor Nakornchaisri, Nakornpathom
Tel: (034) 389-3333
There is a small wiki on the temple here:
http://en.wikipedia.org/wiki/Wat_Bang_PhraHope this helps...
Something I shot there during the tattoo festival on March 10, 2001:

Hope you have fun and get some good shots there this year.