It's good that you noticed and they're actually gonna get into that, but its really only been three or four days. You've only seen nightfall three times:
1. the night they spent at Kukuaku's
2. the night when Aizen was murdered and Ichigo was healed after the Renji fight
3. Ishida fought Mayuri, and Ichigo was healed at Yoruichi's after his fight with Kenpachi.

Ever thing else happened at the same times, but because it was introduced sequentally its harder to tell, in the manga they sorta stagger the other things in a bit so it's easier to tell they're all happening at the same time.