Isaac (the son of promise) actually means "he laughs".
It says, there is a time to weep and a time to laugh (Ecc 3:4) even as Jesus said, blessed are those who weep (now) for ye shall laugh (Luke 6:21) but woe to you who laugh now for ye shall weep (Luke 6:25) is an expression of those times.
Jesus himself wept, he being a man of sorrows, aquainted with grief. He was laughed at and mocked, But through that and for the joy set before him he endured the cross where it speaks of him that sitteth in the heavens (at that time) shall laugh (and in that particular case) would have them in derision.
Psalm 2:4 He that sitteth in the heavens shall laugh:
the Lord shall have them in derision
That might also show the context in which our Lord shall laugh (and at what)
Heb 12:2 Looking unto Jesus the author and finisher of our faith;
who for the joy that was set before him endured the cross,
despising the shame, and is set down at the right hand of the throne of God.
God having fulfilled the second psalm Acts 13:33 and again (His turn to laugh)
Psalm 2:4 He that sitteth in the heavens shall laugh:
the Lord shall have them in derision.
Gen 17:19 God said, Sarah thy wife shall bear thee a son indeed;
and thou shalt call his name Isaac:
Isaac = "he laughs"
...and I will establish my covenant with him for an everlasting covenant,
and with his seed after him.
And...
Gal 3:29 And if ye be Christ's, then are ye Abraham's seed,
and heirs accordingto the promise
And Jesus said,
Luke 6:21 Blessed are ye that weep now:
for ye shall laugh.
And again,
Gal 4:28 Now we, brethren, as Isaac was,
are the children of promise.
Sarah laughed, just at the wrong time.
There is a time to laugh. Although, it's probably not the best time when at what God is promising
Gen 18:13 And the LORD said unto Abraham, Wherefore did Sarah laugh, saying,
Shall I of a surety bear a child, which am old?
Gen 18:15 Then Sarah denied, saying, I laughed not; for she was afraid.
And he said, Nay; but thou didst laugh.
So at the apointed time Sarah would have a son (whose name means he laughs)
And when Isaac was born Sarah says,
Gen 21:6 And Sarah said, God hath made me to laugh,
so that all that hear will laugh with me.
Isaac being the one mocked and persecuted of the bondwomans son, but even so Paul said is it with those born after the Spirit (in relation to those born after the flesh)
Laughter here as well,
Ecc 7:7 For as the crackling of thorns under a pot,
so is the laughter of the fool: this also is vanity.