When to read new spring is a question that comes up with many people starting the Wheel of Time. The prequel novella takes place before the main series begins, and so some new readers want to know if they can skip it altogether.
The answer is no, you have to read it at some point in order to fully understand the rest of the books. The book reveals the backstory of Moiraine Damodred and Lan Mandragoran, two major characters in the series. It's also an important part of the story that sets up the major reveals in the last books in the series, Knife of Dreams and beyond.
The book takes place near the end of the Aiel War, just before the arrival of the Dragon Reborn in Tar Valon. It follows Moiraine and Siuan as they train as Accepted in the White Tower and begin their search for the Dragon Reborn. It also reveals how they came to bond together.
It's also a good way to get a feel for the world of the main series and how it plays out before you start reading New Spring. That will give you a sense of the gravity of the plot, the different political powers, all the Ajahs, and more.
It's not really possible to read New Spring before the fifth book in the main series, but you can fit it in after that. It will make more sense then, and you'll already be familiar with the main series' lore, and have enough curiosity about Moiraine and Lan to enjoy the prequel.