This event is an excellent candidate for a temporary cellular deployment, engineered and operated by a carrier. Typically, they will not charge for the service because of the extra call revenue and to avoid bad publicity from poor coverage or capacity. Unfortunately, you would need to engage both Verizon and AT&T to provide service for their own customers.
I recommend not deploying a cellular repeater in this case because there is likely insufficient isolation between the surrounding macro network and your event site. Without sufficient isolation, the repeater will impair the operation of the host base station and not even provide significant coverage improvement due to massive multipath problems. If you're interested in a survey, you could use an Android phone with RF Signal Tracker and record the RSCP and Ec/Io of UMTS ("3G") service.
If the carriers are not interested, I recommend deploying your own Wi-Fi network. I cannot offer advice on how many access points you would need, but it will be many. Because Wi-Fi was never meant to be infrastructure and lacks features like uplink scheduling and handover, controlling the footprint of each access point is very important.
This particular set looks like it has licensed the GE logo and are actually from a company called 'Santas Best Craft' in B.C. I'm giving them the benefit of the doubt at the moment since I've not been able to talk to anybody there and that this was a con on the part of a the contract manufacturer that drop shipped them from (you guessed it) China to my local store back in October.
Has anybody else also have so-called "LED" strands? I did purchase a single strand from Phillips last year. It for comparison draws ~5.4 watts for a stand of 70.