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).
Planets have a role to play in your life. Some of them are positive, some of them brings bad karma or events. For example, Saturday is ruled by Saturn. Saturn rules hard work, old people. This planet does not give something, does not allow something, impedes. Therefore, in day ruled by Saturn you should not start something new. But, as we know, Saturday is the most popular wedding day. If we look at divorce statistics, it becomes clear why this is happening.
The other planet, Jupiter, is, on the contrary, very beneficial to marriage. Jupiter rules Thursday. Jupiter gives the divine blessing, helps to expand, lets go along the Dharma path.
Read more about the days and when it's best to start something or avoid starting something new.