I’m assuming you want to use purple moor grass for your skep.
If you are planning to use the tall flowering stems, it’s best to cut them as they are turning colour but while still a bit green at the bottom – September/Octoberish? Keep an eye on it. If you let it get completely dry it will be brittle and difficult to work.
If you are planning to use the leaves, wait till they have turned blonde – probably October but just keep an eye on it. Pull it on a dry day.
The leaves will make a softer skep and are easier to work. The stems make a stronger, tighter skep but are a bit of a challenge especially on the smaller circuits at the start.

Regards,

Daniel.