When applying for a Social Security Number in the US you have to wait 10 days after you arrive before applying, this is due to the timelapse getting on the Social Security system.
If you try and get ahead of the game by applying before you have been in the US for 10 days then your application will take up to 4 weeks as oppose to the 7-10 working days so it’s worthwhile waiting 10 days.
I’ve had the pleasure of applying for mine today and it was pretty painless: fill in a form, wait 20 minutes, get called to a window, clerk inputs data, Done!
Monkeys Forehead 