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.
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