"Can you receive Communion more than once in a day? For example, you can receive at a Saturday mass prior to the vigil and then again at the vigil, but can you receive at two separate weekday masses?"
The simplest answer to your question is “yes”, but with certain conditions. These flow from the 1983 Code of Canon Law, which changed the law regarding the reception of Holy Communion by the faithful. The new Code permits the faithful to receive Communion twice daily under certain circumstances, which are spelled out in two Codes, 917 and 921 #2. These read:
“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 prescriptions of can. 921 #2.” (Canon 917)
“Even if they have received Communion in the same day, those who are in danger of death are strongly urged to receive again.” (Canon 921 #2)
Hence, the following can be deduced from these codes and the official interpretation of them.
1. A member of the faithful can receive Holy Communion twice daily if the person attends two separate Masses on the same day. Thus, the person may receive Communion within Mass in the parish and at a funeral, wedding, or other Mass on the same day. A person can receive Holy Communion at a regular Mass and then at a Vigil Mass on the same day. This is consistent with the Church’s stress on a person’s full and active participation at every Mass that the individual attends. However, it is not permitted to receive Communion a third time at a third Mass on a given day. (AAS 76 (1984) 746)
2. A person can receive Holy Communion within Mass and then receive it again on the same day if the individual is in danger of death and receives the Last Rites including Holy Communion or Holy Viaticum. When in danger of death, the person is strongly urged to receive Holy Communion again. A person may receive Holy Communion administered as Holy Viaticum at any time.
3. If a person received Holy Communion within Mass and later attends a Communion service where Communion is distributed, the individual is not permitted to receive Communion a second time. However, if the person first receives Holy Communion at a Communion service and later has the opportunity to attend Mass, the individual may receive Communion within the celebration of the Mass.