A day is midnight to midnight, except for Sundays and holy days, which by virtue of the anticipated Mass the evening before, are reckoned from sundown the evening before to midnight of the day itself. Sunday does not end at sundown, which is why many churches have Saturday AND Sunday evening Masses. But all other days are midnight to midnight.
As for the reception of communion, why not check what canon law actually says?
The Code of Canon Law (No. 917) stipulates, “A person who has received the Most Holy Eucharist may receive it again on the same day only during the celebration of the Eucharist in which the person participates, with due regard for the prescription of Canon 921, 2.” Following this lead, Canon 921, 2 stipulates, “Even if they have received Communion in the same day, those who are in danger of death are strongly urged to receive again.” Succinctly, a person may receive Holy Communion twice a day.
So, the danger of death aside, one may only receive communion twice in one day, and the second time would have to be during a Mass. In most cases, this means that someone may go to as many Masses as he or she likes, but receive only in the first and second. In a rare case, say, when a relative is brought Viaticum, one may receive it along with the sick person as the first time, then again at Mass a second time. But if the situation were reversed, the relative of the infirmed could receive at Mass the first time, but not with the sick communicant during the second, since that is not at Mass. And this mould apply to a communion service, as well, which is not a celebration of the Eucharist.