John 8:56 [blb] says (relative to the context of this discussion at hand), "Abraham your father rejoiced
in that he should see My day--and he saw it and rejoiced."
As one commentator put it, "The words,
“My day,” are used, as in
Luke 17:22,
for the manifestation of Christ on earth."
[I see a cpl other commentaries also stating that this (John 8:56 verse) refers to Christ's Second Coming
to the earth FOR the promised and prophesied
earthly Millennial Kingdom, rather than His first advent/earthly ministry in the first century...]
Something to consider, anyway, seeing as your John 8:51 verse is in the same context as this John 8:56.

[also noticing that "Abraham" is mentioned 11x in John 8 (in 9 verses... but nowhere else in John)]
^ [just to add to that] The phrase
"rejoice [G5463] and exult [G21]" as used here in
John 8:56 (re: Abraham and Jesus'
"My day")
is used in a few other key places:
--
Matthew 5:10-12, esp v.12 - "10
Blessed are those having been persecuted on account of righteousness, for theirs is
the kingdom of the heavens. 11
Blessed are you when they shall insult you and shall persecute you, and lying shall say all kinds of evil against you
on account of Me. 12
Rejoice [G5463] and exult [G21], because great is your reward
in the heavens; for thus they persecuted the prophets before you." [note: 'the kingdom of the heavens' referring to the promised and prophesied
earthly Millennial Kingdom, when Jesus is present there, commencing upon His "RETURN"
to the earth]
--
1 Peter 4:13 - "But as you have shared in the sufferings of Christ,
rejoice [G5463], so that also in
the revelation of His glory
you may rejoice [G5463], exulting [G21]." [note that wherever the definite article ('the') is used in connection with the word "revelation," it refers to His actual, Personal Presence, as opposed to merely an awareness/understanding of Him]
--
Revelation 19:7 - "
we should rejoice [G5463] and should exult [G21], because the marriage of the Lamb is come [aorist] and His Bride/Wife has made herself ready..."
--
Zephaniah 3:14-15, esp. v.14 - "Sing, O daughter of Zion; shout, O Israel;
rejoice [H8055] and exult [H5937] with all the heart, O daughter of Jerusalem. 15 The Lord hath taken away thy judgments, he hath cast out thine enemy: the king of Israel, even the Lord, is in the midst of thee: thou shalt not see evil any more."
____________
^
EDIT: just to tack onto my post above, a brief quote from my Post #529, Page 27 (of a different thread):
"[adding the passages I've pointed out in the past:
1Jn4:2 "
having come [perfect participle] in flesh"
is distinct from 2Jn1:7's "who confess not Jesus Christ
coming [present participle] in flesh"]" [end of that quoted post]
____________
"…those
not confessing Jesus Christ
coming [present participle]
in flesh."