Currently, new Magic sets are released on Fridays. This is the official release date.
The gaming stores receive the new sets before then, but aren't allowed to sell any of them until the release date.
Official release dates were created as a way to orgainize the simultaneous release of new sets. That is the purpose of a release date.
Homelands was the first set that had simultaneous release on a specific date.
Sets prior to Homelands did not have simultaneous release, which I think means that the gaming stores were allowed to start selling the new set as soon as they received it. Some stores were physicaly closer to the distributors than other stores, and recieved their cards first. Not everyone recieved cards at the same time. The result is availability dates that vary from region to region for the earlier sets. This can explain the slight date differences between the websites. It is also why only a month (without date) is listed for those sets release. It is why there are no pre-releases before Homelands. It is why WOTC can not provide exact release dates (they don't exist).
As mentioned above, it can be difficult to determine some of the earlier sets release dates. However, I've found that distributors were given a date on which to release the cards to stores. Sometimes they did not receive the date until the day before when cards were being sent to them, but dates do exist on the distributor paperwork. These dates were not always available to the public, and the stores would have received the cards after this date due to shipping time from the distributor to the store.
Alpha - On July 16th 1993 the first printed cards arrived at the Origins convention where they were previewed but not sold. Many pre-orders were placed.
On August 1st 1993 there was a release party at Peter's house (the WotC "offices"), during which boxes of Alpha booster packs were opened. That day Dave Howell became the first person to complete a full set of the printed cards.
On August 2nd, Peter leaves on a two week road trip to demonstrate the game in local game stores throughout Oregon, California, Arizona, New Mexico, Colorado, and ending at Gen Con in Wisconsin.
August 5th is the day that distributors are allowed to release to the general public. WotC considers August 5th to be the official release date of Limited Edition.
August 19th to 22nd 1993 Gen Con
August 24th Alpha is sold out
August 25th the last Alpha product arrives at WotC and is sent to the distributors who ordered it.
Beta - WotC was supposed to receive Beta by September 20th, but that was delayed until September 24th and delayed again until Oct 4th 1993. It was shipped to distributors as fast as possible to meet demand. Beta is the second print run of the Limited Edition set, and the "official" release date of Limited Edition is August 5th 1993 (when Alpha became available).
Unlimited - Unlimited was supposed to be released on December 1st 1993, but it was delayed. Booster packs arrived at WotC on December 9th and immediately sent to distributors who started shipping them to stores on December 10th. The release date is December 10th.
Starter decks arrived later, between December 15th to 22nd, probably on the 16th in the same shipment as the Arabian Nights cards.
Arabian Nights - Arabian Nights arrived at WotC on December 16th and immediately sent to distributors who started shipping them to stores on December 17th. The release date is December 17th 1993.
Collector's Edition / International Collector's Edition - Pre-orders sold out in October. Several release dates were announced and missed. Distributors in the USA released this to stores on December 21st 1993. About 1500 "International" sets were sold in the USA. At some point European distributors started receiving product directly instead of shipping it to the USA and back to Europe, so I'm not sure if International sets destined for European distributors could have been released sooner due to the shorter shipping time from Belgium.
Antiquities - Carta Mundi finished printing on Feb 20th 1994. Antiquities was released in Europe around Feb 26th to avoid the costs of warehouse storage during the wait for USA bound cards to arrive. When WotC received the USA bound cards, they staggered the shipping so that each region would receive cards about the same time to prevent East coast stores from ordering through West coast distributors. The distributors were told to release the cards to stores on March 10th 1994, but some released early.
Revised - The release date was either April 9th 1994 or April 11th 1994.
Legends (English) - June 13th 1994
The Dark (English) - August 8th 1994
Edgar / Summer Magic - Sent to print around July 1994 with packs inadvertently distributed in September 1994
Italian Edition (FBB) - late October 1994 at Lucca Games Faire
Fallen Empires - Fallen empires was intended to be released a few weeks earlier, but when the color separations arrived at Carta Mundi, they were too dark and needed redone. The finished cards were received on November 11th and released on November 15th 1994.
French Limited Edition & German Limited Edition (FBB) - March 1995
4th edition (English) - WotC received a very small amount of product (tests or proofs?) at the end of March and pushed for an April release, but production delays postponed the release. WotC sent the cards to distributors on May 2nd with a May 3rd 1995 release date.
Ice Age (English) - There was an Ice Age sealed deck tournament at the Canadian Nationals in Toronto on June 3rd and 4th. Regular release was on June 8th 1995.
Spanish 4th edition black border - released July 9th 1995 at the M:tG Spanish National Championships in Madrid.
Chronicles (English) - Scheduled to be released in late July, the release was delayed until an additional shipment of cards could be received to meet demand. August 7th 1995 release date.
Renaissance (Italian, French, & German) - August 1995
Homelands - Prerelease October 14th 1995 with regular release on October 18th 1995
Alternate 4th edition - sometime between October 9th 1995 when WotC announced the would use USPCC for two product lines, and 1996 when their partnership ended.
Japanese 4th edition black border - Prerelease April 9th 1996 in Shinjuku with regular release the next day April 10th 1996. Work had started on Japanese language cards before Spanish, but the Japanese translation took six months which resulted in a later release than Spanish.
Alliances - Prerelease May 18th & 19th 1996 with regular release on June 11th 1996. Other preview events were held between the prerelease and regular release.
Rivals Quick Start - Released in early July 1996 on or before the 17th.
Chinese 4th edition - released August 23rd 1996 in Taiwan and Hong Kong.