The mitzvah of Mezuzah applies to one who lives in a house. The fact that you now own (or are renting) a new house does not obligate you yet. Hence, you should wait until you enter the house to live there, and right at that point is the ideal time to install the Mezuzot, or the sooner the better. It is not necessary for the rest of the family to be there when you install the Mezuzot, so if they cannot be there then, it is preferable not to wait. However, it adds to beauty of the mitzvah if you can arrange for the family to be present when you arrive.
(See also question “Affixing mezuzah before installation of doors.”)
 Agur B’Ohalecha 2:11; with additions as heard from the author. Compiled by Shmuel Kraines.