No matter how lucky you think you are...
No matter what the Farmer's Almanac says. ..
No matter what the weather man says...you always prepare for the worst case scenario when it comes to having an outdoor wedding.
A friend told me about a wedding in New Hampshire he went to recently. The wedding was in early/mid-May so the couple -- who had a tent wedding -- decided that they didn't need to rent heaters becasue they assumed that in May the weather would be nice.
Well, unfortunately for them and their guests it was quite a chilly night and their guests ended up hanging out in a house (with heat) on the property because it was too cold to be outdoors without heaters.
Whatever your budget is you do not want to skimp on tents, heaters and flooring for an outdoor wedding. You'll risk a chance of your guests leaving early or ducking out to some place warmer.
Another tip is to plan for nice weather but to have a plan b just to be safe. Your plan b will require that you put a hold on everything from flooring to tent flaps and heaters (or AC units if it's too hot) and then you have to let your rental company know several days before the wedding date if you want to use those items.
And remember, it's good luck if it rains on your wedding day.