I start ferts the day I plant.
Here is my rational: my primary goal with a new planted tank is to get the plants going asap. With a given set of plants, light, and substrate I really do not know if plants are short on any nutrients without extensive water testing day after day. So, why not give them everything from the start? I do go either light or heavy with dosage: more light = more ferts, rich substrate = less ferts. I start at 1/2 of recommended EI and adjust up or down based on the tank.
Bump: dosage wise, substrates can get tricky and you need to think a bit. Ask if your specific substrate is likely to leach nutrients into the water column and and what types of plant you have. As an example, Aqua Soil leaches a lot at the start and if I mostly have stem plants then I go light on macros and heavier on micros. If you have a capped soil with mostly crypts then I wait a week before fertilizing and check on how plants are doing. If you have capped soil with mostly stems, then go 1/2 EI to start.