Under canon law, Friday runs from midnight to midnight. On the liturgical calendar, the celebration of the Immaculate Conception begins at 4pm or so. Since this is a private devotion, there is no authority to determine which applies, merely conventional wisdom as to how the instructions to St. Mary Margaret are to be followed.
From
this source, we get a description of the origin of this devotion by
And He told her what He required of her; first, to receive Him in the Blessed Sacrament; second, always to do so without fail on the first Friday of the month; third, every week on the night of Thursday or Friday, to lie prostrate for an hour, her face to the earth, to share the “mortal sadness” which He had chosen for His own in the Garden of Olives. These three commands she was to fulfil within the limits of the obedience owed to her superiors.
So from the start there is a recognition that there will be limits in following this instruction. One obvious limit is Good Friday, when there are no masses.
Here is another description, which seems to allow a much looser interpretation of when this must be done - not sure how much stock to put in this, but for what it’s worth:
As the object of this devotion is to inflame our hearts with an ardent love for Jesus, and to repair thereby, as far as lies in our power, all the outrages which are daily committed against the Most Holy Sacrament of the Altar, it is evident that these exercises are not confined to any particular day. Jesus is equally deserving of our love at all times; and as this most loving Savior is daily and hourly loaded with insults and cruelly treated by His creatures, it is but just that we should strive each day to make an the reparation in our power. Those, therefore, who are prevented from practicing this devotion on the first Friday can do so on any other day during the month. In the same manner they may offer the first Communion of each month for this intention, consecrating the whole day to the honor and glory of the Sacred Heart, and performing in the same spirit all the pious exercises they were unable to accomplish on the first Friday.
Notice, however, that they say it may be done on other days if one is
prevented on Friday. If you are not prevented, you should certainly do this on Friday rather than another day. Likewise, unless you are unable, it would be much more appropriate to attend a mass of the Friday or a special Sacred Heart mass, rather than the Immaculate Conception mass.
If you are unable to attend one earlier in the day, I think an IC mass would be OK. However, since the devotion to begin with was an “extra” mass beyond the normal obligatory masses, I think in that case you should try to attend a second IC mass on Saturday.
Just my opinion - I’ll be interested to see what others say.
