Flydream
All your coverage and everything to do with your legal employment status officially starts on your join date. However you will find that in harsh reality until you have a residents visa/ official ability to work legally you are at the companys discretion.
Though you arrive on a temporary pink A4 employment visa, until you are scanned, finger printed and medicalled and then they have actually put the visa in your passport you will find that you have no access to any part of eGovernment so though by any reasonable standard you are legally employed you would be unlikley to be able to receive any benefit from that status until you have the visa.
What does this all mean, no bank will touch you, cannot register a car, cannot insure car, cannot sponsor family, cannot get driving licence, cannot register children at school etc, etc
That said they are pretty quick now with your visa.i.e 2-3 weeks maximum.
With respect to joining I would come 2-3 days early like advised above to get your bearings unless you are already familiar with Dubai and also bear in mind if you are one of our 2-3 groups of July joiners that you are joining in Ramadam so many things like banks etc will be closed when you arrive here and no one will/can help you coupled with you will be working 10-12 hour days for your first 3-4 weeks every weekeday. So please make sure that you bring enough cash or access to your foregin bank as possible as even when you get joining allowances etc you will find that you are forking out for a lot of initial set up.
If you need help with schools and the like then PM me like High Energy said. If you want British curriculum then I can help more than if you want Aussie or US etc. We only would send our children to 5 schools and therefore to secure a place had to pay 25,000 AED debenture for each child before they would even let us have a place. This is on top of registration fees and assesment fees, (bear in mind your child can fail entry standard assesment and you will loose the place)
On top of this you need to get a car, pay house deposits, agency fees again for housing, likely school fees deposit (at least 10% of your annual fees per child) all in all we needed just under 200,000 AED of cash before my wife had even gotten on the plane from UK. So that she arrived into our house, with the kids and school places and with a car to drive.
Last but not least, if your wife has never driven on this side of the road before that may be a factor with school runs to take into account!
Good luck!