The ideal time for a newborn session is usually the first 14 days of the baby's life.
During this period babies still sleep a lot, are calmer and easier to position, so the photos also tend to have a sleepier, softer, newborn feel.
Of course, it's no problem at all if you can only come after this period. Beautiful photos can be made then too — simply in a slightly different mood. With more alert, active babies, more eye-contact, movement-filled, natural moments are born.