Release dates vary by country - or, to be more precise, according to sales territories.
It is not uncommon to have a UK release date on a Monday and the US release date as the Tuesday. Even then, the same album might be released in Germany and Italy on the preceding Friday. Often. it just fits in with (or used to fit in with) how the album charts are compiled - which day are the sales counted to establish the chart listing. A well-timed release might garner large enough sales in a day or two to get into the chart and just getting into the chart, in itself, might gain a few more sales for the album.
Now that all of this is compturised, things are a lot more strict. It used to be less strict. If Monday was the official release date here in the UK, the record shops would often get their delivery on the Saturday (sometimes on the Friday) beforehand. If you knew your record retailer personally and if he knew you bought almost everything by a particular artist, then he would sometimes let you have the album prior to the official release date and put the sale through his sytem on the Monday. You got the record early, he got his sale (you didn't go somewhere else for it) and the record company notched up another contribution towards potential chart success.