Superficially, D-Day (1965) has only very minor physical component changes from the 1961 versions. The units are identical; the map nearly so (4 hexes between the Rhine and the Meuse in Holland are now land). D-Day 65, however, incorporates significant rule differences from its predecessor. D-day 61A and 61B have 4-page rule booklets, but the rules in D-Day 65 consist of a 4 page booklet for the basic game plus a 12-page "Battle Manual" containing rules, examples of play and, commentary.
The object (Allies enter and remain in Germany in force before the 50th turn) and operational mechanics (select one invasion area among 7, invade and get ashore in strength, then advance to Germany -- possibly aided by a second invasion) are largely the same as in the predecessor. Game tactics in general are the same as in all AH land wargames up to this date (rigid ZOCs, same CRT, try to get 3:1 attacks, try to surround defender with ZOCs, "soak off"if necessary). There are some rule differences that have a major effect on game play. Among the changes are:
1. A rigid 17-hex supply line length is imposed on the allies (rendering use of S France as an invasion area bizarre if not, as some claimed, totally useless).
2. The same CRT is used as in all AH games of this period, but low odds round in favor of defender instead of toward 1:1 as in the 1961A games.
3. River defense bonus applies only if all attackers attack through river hexsides (in the 61A version, the bonus also applied if attacker has moved 1 hex across a river into the defender's ZOC). River defense deployment is considerably different as a consequence.
4. The tournament game added "strategic air attacks" to the Allied arsenal: 8 attacks per game in each of which (usually) 2 or 3 German divisions would be destroyed. (Some refer to these, sarcastically, as "nukes").
5. Allies now get 2 replacement points/turn starting turn 9 (formerly none), but Germans can no longer replace HQ or static divisions (leading to shortages in the number of units on the board later in the game).
6. Units in fortresses now are always in supply.
7. Retreat path of 2 hexes through Mtns now is prohibited, but retreats across rivers are allowed (as in 61A, but unlike 61B).
This is not an exhaustive list: there are numerous other minor rule changes and clarifications in D-Day 65. By the time D-Day 65 came out, the Avalon Hill General was in publication and there were frequent "official" changes to the rules promulgated in a question and answer section of the General, especially as regards details of supply .
Any of the versions are playable (if players can agree on a few rule interpretations or fixes) and even today are enjoyable, so long as you are not expecting a 2000-era game system in a 1965-era game.
Source: E_T_Lee, personal comparison of components and rules