I am not sure where this waiting idea comes from as it’s in another question too.
So, plant winter veg as soon as your potatoes and onions are finished, the same day is good.Soil with plants in, during the growing season, is healthier, though it’s not always possible especially in spring as one waits to plant warmth-loving plants.
In summer, no need to wait and every day counts as light and warmth lessen.
