Hi
@Bromage, welcome to Dementia Talking Point.
I'd not explain anything, just organise the home and then take her there. If you say anything, even if it is dressed up as a holiday or due to work needing to be done on the house she is likely to say no and, even if she forgets what you've said, feel anxious about something.
Find a home that you are happy with, they are not all alike and some will be a better fit for your wife than others. On the day just say you are going out for tea or on a trip and just take her there. It is easier if more than one person is involved, and it might be best if it isn't you. My mother in law moved into care last September. The son who was her principle carer didn't want to be involved in moving her, so my husband and her elder daughter did it. The daughter went ahead to the care home to get the room looking as much like her room at home as possible while my husband took his mother in law for a drive. They sat looking at a view for an hour while playing classical music, both things she liked. They arrived at the home where a big fuss was made of her and they made sure she had a cup of her favourite Earl Grey tea. I don't think she realises most of the time that she is no longer at home.
I'm sure others will be along soon with their tips and suggestions. This is a very friendly and supportive place, and you'll get lots of help and advice here.