You won't find a law that says "An employer can give out a postdated paycheck ahead of the pay date." because the law doesn't work that way.
The basic principle of law is that anyone can do anything they want UNLESS there is a law that specifically says they can't. The wage order for customer service workers is 8 CCR 11040 (Which you can easily Google) and nowhere in that order does it prohibit that. Since it's not prohibited, it's legal.
"According to some others whom I have posed this question, receipt of a paycheck whether or not it is post dated for the next day or two constitutes 'constructive receipt' with the right of the bearer to cash it."
It's perfectly 'legal' for you to have deposited or cashed the check once they gave it to you. However, the financial risk of doing so is on you.
If they had in fact not funded their payroll account until the paydate (and they are under no obligation to do so) then when your bank submitted it to their bank for collection, it would have bounced. Your bank would have then charged you bounce fees.
If you didn't deposit it, but just cashed it at a bank where you don't have an account for them to charge, they'd have set a collector on you for their fees. Had you written checks against that money, or had ATM transactions go through because the bank had allowed you access to the money, those would also have cost you fees.
Additionally, of course, you were told by your boss not to cash it and you did. Regardless of whether it was legal of you to do so or not, they still could choose to fire you for that. The fact that it's been five days doesn't mean that it's a closed issue. My company can't authorize the purchase of a paperclip in five days, let alone process a firing.
Finally, next time pay-day falls on a day when many people are off, the company may just say "Sorry, everyone, you'll have to come in on your day off to pick up your paychecks. We can't give them to you the day before like we used to because Jane Doe over in customer service cashed hers early last time, so we can no longer trust people not to." That will make you very popular with your co-workers.