When should you thatch?

De-thatching in early spring is a bad idea.

The ground is very wet in the early spring.

Try to avoid walking on it at this time. It can compact the soil which will damage the soil structure, making it harder for your grass to grow.

If you have a significant thatch layer built up the best time to remove it is at the end of summer. Ideally right before you aerate and over seed.