When Do I Need a Mezuzah?

When purchasing a home, affix the mezuzahs, with the blessing, immediately upon moving in.

If you are renting the home or apartment the obligation to install a Mezuzah applies only after thirty days. However it is obviously preferred not to be without a Mezuzah, therefore many have the custom to affix all of the mezuzahs immediately upon occupancy, without reciting the blessing. Once the 30-day period has elapsed, one of the mezuzahs (from a room that has a door) should be removed, checked and replaced. That mezuzah may also be replaced with a new or upgraded one. The proper blessing is then recited on the new Mezuzah while having in mind the other Mezuzahs as well.

Some have the custom of temporarily affixing the mezuzahs immediately upon occupancy using tape, and nailing them in on the 31st day. The first mezuzah that is nailed in should be on a room that has a door. The blessing is recited at that time.

The above applies only outside the Land of Israel. In the Holy Land, the mezuzahs are affixed immediately with a blessing.