In Book 20 Odysseus prays to Zeus, asking for two omens, one from the mouth of someone in the house and one from the outside world. Zeus responds with a clap of thunder even though the day is sunny. One of the women outside grinding wheat hears the thunder and says she hopes it is a sign from Zeus that the day is the last one she will have to grind wheat to prepare bread for the suitors. When he hears her comment, Odysseus is assured that Zeus has answered his prayer. 

What omens did Zeus send to signal Odysseus's success before the bow test?

Prior to the assault on the suitors, Odysseus harbors doubts that he and Telemachus will have the fortitude to slay such a great multitude of men. He asks Zeus to send him a good omen, and the god grants his request. First, he sends a clap of thunder. Immediately afterward, Odysseus hears a servant curse the suitors and hope that this will be the last day she will have to prepare bread for them. Finally—after Telemachus threatens to kill one of the suitors for insulting the disguised Odysseus and Athena causes them to laugh at him—Theoclymenus recognizes that the suitors look ghastly. He correctly takes this as an omen that evil will soon come to the suitors. It is not long before these omens come to fruition.