I have never grown pickling onions - usually I pickle the small shallots - but as others have said bulbing onions are ready to harvest when the foliage begins to brown and shrivel.
I find it best to then lift them partly away from the soil at that point if the weather is dry and leave then to ripen outside, but if its wet - as with me now - they are lifted and put in a light airy barn or, if there is space, in the polytunnel or greenhouse.
The ripening helps to sweeten them up a bit and brings out the flavour. Then you can pickle them at the end of Oct or during Nov, so they will be nice and crisp for Christmas