Roosters crow when dawn comes. Yeah yeah, you knew that. But that well-known fact takes on enormous significance for all the Middle-earth Good Guys.
Dawn becomes more and more unreliable as the troops of Sauron start advancing on Gondor. Pippin notices that each day, the dawn grows darker and darker until it is utterly black. The power of Sauron is so great that it can actually blot out the sun itself.
However, Sauron's darkness is not perfect, and there are several moments when breaks appear in his clouds. First, with the arrival of the Riders of Rohan to reinforce Minas Tirith, the clouds seem to lighten. And then, as Frodo and Sam struggle through Mordor, Sam points out: "Look at it, Mr. Frodo! [...] Look at it! The wind's changed. Something's happening. He's not having it all his own way. His darkness is breaking up out in the world there" (6.2.29).
This symbolism of night versus day is pretty basic stuff in literary terms. Light is good; dark is bad; yadda yadda yadda. Even so, we have to say that Tolkien does a beautiful job of using the struggle between darkness and dawn to represent the conflicts between Sauron and the side of good. When the Riders of Rohan arrive to help Minas Tirith in its most desperate hour, their appearance accompanies a break in the clouds. Dawn comes at last, and a rooster crows to greet it:
And at that very moment, away behind in some courtyard of the City, a cock crowed. Shrill and clear he crowed, reckoning nothing of wizardry or war, welcoming only the morning that in the sky far above the shadows of death was coming with dawn. (5.14.198)
This contrast between the horrible forces of Sauron blotting out the light and something as homely and natural as a rooster crowing at dawn gives us hope that this will be a turning point in the series' battle against evil. The awesome everyday will return to Middle-earth.
To reinforce this connection between dawn and goodness, Aragorn deliberately waits until first light before entering Minas Tirith as its king following the fall of Sauron: "And there in the midst of the fields they set up their pavillions and awaited the morning; for it was the Eve of May, and the King would enter his gates with the rising of the Sun" (6.4.75).
Talk about making an entrance. That Aragorn always had a knack for timing.