So we're getting married outside in 19 days. Right now, the extended forecast is calling for partly cloudy skies and a HIGH of 59 and low of 39.

Historical average high/low for that day is 78/47. I'm aware the forecast will change pretty much daily from here til the wedding, but it's still got me worried.
We do have a back up reception location in the same area that's indoors, but it's so ugly and I'd be so incredibly disappointed if we had to move inside. We'll have a few space heaters, but at what point do they become inadequate? I was also thinking about buying something like this:
http://pinterest.com/pin/249175791852595441/ So it's got me wondering, at what temperature do we decide that it's just too cold to stay outside?