When is M&S' 3 for 2 uniform sale usually on ?

[Sorrento]

DD is insisting on wearing pinnies not skirts and is flashing her draws, but it's still too cold for summer dresses.
I'm going to have to buy more winter stuff sob sob

M&S have a 50% sale on starting today! I got dd1 some navy pinnies for £4 each

Thank you, I saved £21 on pinafores and bought a £400 bed, bargain