Generally speaking, if a priest is present or one is available, then Mass should be offered. Even if a priest has already said Mass that day, for pastoral reasons, he can binate and say another Mass. As far as I know, bishops generally make this approval so that priests don’t have to get permission every time this happens.
Rome has given directive regarding these larger assemblies for communion outside of Mass (there is also a shorter rite in both the OF and EF) for the faithful to receive communion outsid of Mass. It was more common prior to Vatican II for communion to be received outside of Mass because Mass was only permitted to be offered before noon. In order for those who couldn’t make it to daily Mass in the morning, but still needed to receive communion for First Fridays and First Saturdays Devotions.