The Rebbe's guidance on bris milah:
Its meaning. The holy soul begins to enter the child at the time of the bris (IK 2776). On welcoming a newborn son with the bris, see IK 909.
Timing and health. A bris takes place only when the child is healthy enough for it (IK 2003); on bris timing, see also IK 896. Milah is one mitzvah with three parts, and it is not limited in time once the proper day has passed (IK 8909).
Doing it properly. Consult a qualified Rav on all the details of the bris (IK 10286), and take care with periah and metzitzah according to halachah (IK 5410). On reciting Shehecheyanu at a bris, see IK 7727; on the custom of two chairs, see IK 870; on the sandak and naming the baby, see IK 4248. When there is doubt about a child's Jewish status, consult a senior halachic authority before the bris (IK 9076).
See also: Parenting – Naming a baby and the upsherin; Family life – Hoping for children.