Worship
Griha Pravesh and Vastu Shanti
The first entering of a new or newly built house.
When
The muhurat comes from your acharya, usually by a chosen weekday and lagna.
Griha pravesh often includes vastu shanti, a Ganesh puja, havan, and a kalash brought in. Many houses boil milk as a wish for plenty, and cross the threshold with the right foot. The details change by region.
A new flat or an old house opened after years — both are a sankalp that the place be lived in peacefully. Do not buy fear in the name of vastu. If a fault is named, test it with your own acharya.
This is a general introduction. Sequence, materials, and timing change with family, region, and shakha. Your priest and your own custom have the last word.