The final scene of Act II opens with "so smile the heavens upon this holy act...." Of what holy act is Friar speaking?What does this foreshadow as...

In the Prologue to Act I of Romeo and Juliet
the Chorus states,

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A pair of star=cross'd lovers take their
life;

Who misadventured piteous
overthrows

Do with their death bury their
parents' strife.
(1.1.6-7)

The marriage of
Romeo to Juliet is the second of their "misadventured piteous overthrows"--a series of
events that keep them apart. After the words of Friar
Laurence,

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these violent delights have violent
ends,

And, in their triumph die, like fire and
powder,

Which as they kiss, consume.
(2.6.9-11)

Romeo commits his
second act of "misadventured overthrow"--the first is invasion of the party and meeting
with Juliet--as he encounters Tybalt and seeks to ameliorate the tense conflict between
Tybalt and Mercutio.  His well-meaning declaration of love for Tybalt now that he is
related to the Capulets through marriage is misunderstood and backfires as it causes
Tybalt to become so enraged that he stabs Mercutio under Romeo's
arm.

As a result of his friend's angry death and curse
upon "both your houses," Romeo loses his dear friend, whom he has tried to save from
harm,  Romeo's act of love for Tybalt becomes overthrown by the insidious act of
Tybalt's having stabbed Mercutio and Romeo having, then, killed Tybalt--"a piteous
overthrow," indeed.