You can receive communion during mass twice in a day. You must participate in each mass; you cannot participate in one mass and then just show up for communion in the second (not that I suspect you would).

This is actually a matter of Canon Law.

[quote:15ejgx16][b:15ejgx16]Can. 917[/b:15ejgx16]
A person who has already received the Most Holy Eucharist can receive it a second time on the same day only within the eucharistic celebration in which the person participates, without prejudice to the prescript of can. 921, §2.[/quote:15ejgx16]
You can receive communion three times in one day only if you go to two masses and receive viaticum during the Anointing of the Sick.

[quote:15ejgx16][b:15ejgx16]Can. 921, §2[/b:15ejgx16]
Even if they have received Communion in the same day, those who are in danger of death are strongly urged to receive again.[/quote:15ejgx16]