Why are concrete slab expansion joints important?
Expansion control joints are placed in concrete slabs to control random cracking. It allows movement to compensate for inevitable changes in temperature and drying shrinkage. In general, your expansion joints should be a minimum of 1/4 the depth and up to 1/3 the depth for commercial and industrial projects to the concrete slab.
Unfortunately, there is no exact right time to cut your expansion joints as it will depend on how fast the concrete sets up. Cuts should be made as soon as the concrete is sufficiently hard to withstand the energy of sawing and resist tearing/raveling (dislodging concrete particles). The ideal time is usually within 6-18 hours after the concrete has been poured.