I actually phoned them up to ask this a week or two ago. The lady on the Phone said usually they "arrive by" that date. She said you'll get a notification with a tracking number and the payment will be taken at the same time. It's particularly important for me to know because it's getting delivered to my work address, but then if it doesn't come until the next week I'm away and I don't want it kicking around there for a week!
I decided if I didn't get a tracking number by 27th I'd phone them up to change the delivery address.