The process is:
1) You enrol at uni
2) The uni confirms to SF that you have turned up and enrolled
3) SF release your funds
4) Your funds move through the banking system into your bank account
Each one of those stages can potentially involve delays. The date given by SF is the eariest possible date on which they will release your funds. If your uni enrolment date is after that, then your payment will be later than the date given by SF.
As an example using your case, if you manage to enrol on Fri 18th, the earliest that the uni is likely to inform SF is Mon 21st. Then you might get your funding sometime that week. In reality it all depends on how soon after your results you can enrol, how soon after that your uni tell SF, and how quickly SF release your funds. Nothing is likely to happen over weekends. Make sure you have enough of your own money to cover you for the first couple of weeks, just in case.