There are seven days during the week. Each day is governed by particular planet (graha) and planets are governed by a particular numbers. In vedic numerology week starts with Sunday. In some traditions but not in vedic tradition week starts with monday. From point of view of vedic tradition that is not correct.
|Day of week||Planet||Graha||Number|
In some theories Sunday and Monday operates also numbers 4 for Sunday and 7 for Monday. It happens when Moon is waning (dark half of the month). Let's say - person was born on Sunday with Crescent moon. In this case he's number of the day will be 4.
Number 4 is meaning shadow planed called Rahu (Lunar North node).
Number 7 is meaning shadow planed called Ketu (Lunar South node).