The best day is Wednesday or Thursday, in the 2nd, 4th, 5th, 9th or 11th waxing or waning moon. There are also several dates in the year that are bad luck for this ceremony like โลกาวินาศ (LoKaViNas) and อุบาทว์(OoBaat) days. As for the location, the spirit house should be at the north side of the main building, and facing the north. If north is not available, the northeast would be your next choice. No matter which location you choose, you need to be sure that it's not under the shadow of the house, or vice versa.