I'm not very familiar with doing this but in my opinion I would do it in one day. It would insure there will be a lot of people there. Nothing worse than a open house party with only a hand full of people just standing around. Make it one big event.
Make sure you have finger food and drinks too! Do some demonstrations of what you do to make it interesting.
One day, lots of people, food, prizes, etc. Keep everyone there as long as possible. Do the actual ribbon cutting at hour 2 of the 3 hour event. We had a well known local cafe/catering company do the food (burgers and dogs) so we didn't have to worry about that. Had a keg and drinks.
My best/worst idea for a demonstration was to wait until hour one when we had a good number of people, took a 12 foot ladder to the edge of the crown, snapped a picture with my camera phone, emailed it to the design computer, put together a banner, printed it, finished it, and showed it to the crowd 20 minutes later. People were amazed how fast it was possible to pull that off and loved finding themselves in the picture. I put the date on it and a "thanks for coming" message and had it on the fence as everyone left.
Ditto... i do alot with the chamber and attend these functions. All in one day but you can continue the celebration though out the month with an "open house" and give-aways and leave the banner up etc.