The second time is when Jem and Scout are walking on their way to the Halloween pageant. In order to get to the schoolhouse they must cross a forested area. As they come up to it, "High above us a solitary mocker poured out his repertoire in blissful unawareness of whose tree he was in, plunging from the shrill kee-kee of the sunflower bird to the irascible qua-ack of the blue jay, to the sad lament of 'Poor Will, Poor Will, Poor Will."
In this here the mention of mockingbirds seems to herald what is to come. After all, Scout does say that it was the beginning of her longest journey ever with Jem. Indeed, an important event does happen at the end of this pageant sequence, and this ominous mockingbird repertoire seems to signal that it is coming.
Here it seems almost as if the mockingbird's song is in sequences. The "blissful unawareness" may be related to when Scout and Jem are walking to the pageant unaware of Cecil Jacobs' scaring them and how the pageant is going to go.
Secondly, the "irascible qua-ack" of bluejays, which is generally speaking relatively jolly and happy, stands for the good time they have at the pageant.
Lastly, the "Poor Will" thing means when Scout and Jem are returning home, with the atmosphere lonely and sad.