what i took from an earlier discussion on the subject is that in the liturgy has changed, then the day has changed, that the day is based on the Liturgy
i will have to look for that and i know there were also citations on that thread
Canon Law specifies that things in Canon Law…like that one can receive a second time in a day if second is at Mass one participates in…are to be understood on the normal midnight to midnight understanding unless a Particular Canon says otherwise…and this one does not specify anything special.
Code of Canon Law (Latin Code) States:
Can. 202 §1.** In law, a day is understood as a period consisting of 24 continuous hours and begins at midnight unless other provision is expressly made**; a week is a period of 7 days; a month is a period of 30 days, and a year is a period of 365 days unless a month and a year are said to be taken as they are in the calendar.
Can. 917 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.
If it meant to include the vigil in this…it would have to expressly state this…which it does not…so ergo it is to be understood as the normal 24 hour day. Midnight to Midnight.
and further note regarding fulfilling ones Sunday Obligation:
Can. 1248 §1. A person who assists at a Mass celebrated anywhere in a Catholic rite either on the feast day itself or in the evening of the
preceding day satisfies the obligation of participating in the Mass.
it even expressly states that the vigil is the
preceding day…not the same day as Sunday or the Holy day…(in regards to Communion for sure)…it is the vigil!