84 At the same time, all is not recorded, as John plainly declares. For He had frequent intercourse with His disciples during the forty days which preceded His ascension into heaven. He had not, however, showed Himself to them throughout all these forty days without interruption. For John tells us, that after the first day of His resurrection-life, there elapsed other eight days, at the end of which space He appeared to them again. The appearance which is identified [in John] as the third— namely, the one by the sea of Tiberias— may perhaps have taken place on an immediately succeeding day; for there is nothing antagonistic to that.
And then He showed Himself when it seemed the proper time to Him, as He had appointed with them (which appointment had also been conveyed in the previous prophetic announcement) to go before them into Galilee. And all throughout these forty days, He appeared on occasions, and to individuals, and in modes, just as He was minded. To these appearances Peter alludes when, in the discourse which he delivered before Cornelius and those who were withhim, he says, “Even to us who ate and drank with Him after He rose from the dead, for the space of forty days.” It is not meant, however, that they had eaten and drunk with Him daily throughout these forty days.
For that would be contrary to John's statement, who has interposed the space of eight days, during which He was not seen, and makes His third appearance take place by the sea of Tiberias. At the same time, even although He [should be supposed to have] manifested Himself to them and lived with them every day after that period, that would not come into antagonism with anything in the narrative. And, perhaps, this expression, “for the space of forty days,” which is equivalent to four times ten, and may thus sustain a mystical reference to the whole world or the whole temporal age, has been used just because those first ten days, within which the said eight fall, may not incongruously be reckoned, in accordance with the practice of the Scriptures, on the principle of dealing with the part in general terms as the whole.
Source: The Harmony of the Gospels (New Advent)