D-Day (1961) was one of the first hex-based wargames (the other three published the same year being Gettysburg(1961 hex version) and Chancellorsville, which shared a common game system, and Civil War, a simplified strategic ACW game).
The map covers all of France, the low countries and part of Germany east of the Rhine. The style of AH maps of in 1961 was for single terrain-type hexes each monotone-colored or patterned, with rivers, boundaries and coastlines following hexsides. The result was criticized by some as being artistically unappealing, but on the other hand was quite unambiguous with regard to terrain type. A map difference that distinguishes the 1961 from the 1965 version is that in the 1961 version a 4-hex area between the Rhine and Maas Rivers are depicted as a lake, whereas they are clear terrain in the '65 version.
There is a plethora of defense-doubled terrain on the map: mountains, Westwall fortifications, cities and smaller towns. In the case of rivers, a defender is doubled if it is attacked through river hexsides or if attacker has crossed a river into the defender’s zone of control. The second condition disappeared in the next edition of the game, considerably changing detailed deployment and tactics.
The Allies object is to cross the Rhine in force before the 50th turn. They do this by selecting one of seven invasion areas, invade it and get ashore in strength, then advance to Germany -- possibly aided by a second invasion. The German player attempts to prevent this first by deploying all his forces to protect the possible landing areas, then by either defeating the invasions(s) at the beach, or failing that establishing one or more defensive lines in the interior, thereby imposing an attrition ratio on the Allies that eventually renders him impotent.
Units are mostly division-sized. For the Allies, these consist of motorized infantry and armored divisions plus a few paratroop units with airdrop capability. For the Germans, the only motorized units are Panzer and Panzer Grenadier units. The Germans have several varieties of infantry, many being weak, slow-moving "static" divisions that must initially be deployed on coastal hexes. Other than movement factor and unit combat strengths, there is no functional difference between unit types (beyond the paratroop and static division effects previously mentioned).
The core game system and tactics in general are the same as in all AH land wargames up to this date and for a few years beyond (rigid ZOCs, same CRT, try to get 3:1 attacks, try to surround defender with ZOCs, sacrificial "soak offs" to obtain better odds in other attacks). A major difference in combat resolution in the 1961 games, however, is that inexact low odds combat ratios round toward 1:1, whereas in 1962 and later games, low odds rounded in favor of the defender (i.e., 5:8 in this game resolved as 1:1 rather than as, in later games, 1:2).
Supply rules are present but simple. The Germans simply trace a line from each unit to the east of the Rhine. The Allies are limited in the number of units ashore by the port capacity captured, and trace their supply to those beaches or ports. Units that cannot trace a supply line are eliminated after two turns of isolation, suffering no other effect until then.
D-Day seemed to be a favorite in its first years, probably because of the appealing historical subject (this game came out only 17 years after the historical event; if kids playing this didn’t remember the events, their parents certainly did) and also to the considerable variety in the course of play brought about by the many combinations of German initial deployments and Allied invasion area choices.
The rules were very short (4 pages), and maybe needed to be a bit longer. There were several significant ambiguities and gaps in the rules, especially in the Allied supply rules. How those are resolved makes a significant difference in the play of the game. Until the advent of publication of the Avalon Hill General in 1964, there was no widespread means of publicizing errata or rule revisions. So every local group of players was compelled (whether they recognized it or not) to develop their own local rule interpretations. Once the AH General began publication, numerous "perfect defenses" were published, showing the continued popularity of the game several years after its publication. Within a year, the 1965 edition was released, which had significant rule changes (see separate version description).
This version is sometimes termed "D-Day 61A" to distinguish it from "D-Day 61B." "D-Day 61B" was published within a year of the original version and contained a revised rules folder, reference folder and combat example card and differed significantly in certain rules. Other functional components were the same. (See separate description)
Description by E_T_Lee; source: personal observations