Crepuscular means occurring during or related to twilight. Crepuscular rays are the sunbeams that occur before sunrise or after sunset. I sometimes call them a sunburst. I've heard them called sunbeams, God beams, or fingers of God. Whatever one calls them, a great sunburst -crepuscular rays- is a glorious thing to see.
Anti-crepuscular is kind of a misnomer, I guess. Anti-crepuscular rays occur at the same time as crepuscular rays, that is, during twilight, but on the opposite horizon. It's almost disconcerting to see sunbeams and the full moon on the same piece of sky.
Crepuscular rays converge towards the sun. Anti-crepuscular rays converge towards the point in the sky opposite the sun – the anti-solar point. The convergence actually is an illusion. The rays are virtually parallel. They only appear to converge. It's the railroad track effect. Parallel lines appear to converge towards a vanishing point at infinity.
Crepuscular rays are a fairly common sight. Anti-crepuscular rays are kind of rare. If you see a sunburst, turn around and maybe you'll see anti-crepuscular rays. And if you do see them, then you can exclaim, “Anti-crepuscular rays!” And it just makes you feel good to say the word. Anti-crepuscular. What a great word. Doncha just love it?