The "President's Day Sale" is a very effective "nice guy" strategy that could (can) be used to win games of Civilization and Civilization II.
I apologize for describing so many game-specific details, as this is Everything2, not a gaming site. If you are interested in the "why" of this article, jump to "What Have We Learned?", at the end. For the rest of you:
In Civilization II, as in most Civ-type games, most of what a player wants come from population (population points). Normally, population growth is quite slow and even painful (as, especially on the more difficult levels, most population points demand extra, expensive measures to pacify the population).
Each population point can be unhappy, merely content, or happy. There are special benefits for having very happy cities with 3 or more population points: concretely, for having no unhappy points in a city, and at least as many happy as contented. The benefits always affect only the specific city or cities, and depend on a civilization's government type. For example, for republics and democracies, the effect is the addition of 1 population point to a city per turn, on every turn after the first one (I think) in which the city is this happy, if there is sufficient surplus food.
This highly desirable state (the equivalent of a 10x or 20x growth multiplier, on average, IIRC) is called "We Love the X Day", e.g. "We Love the President Day," or in player slang, "President's Day" for short.
The President's Day Sale strategy, in its broadest sense (the one I will describe), means planning and playing an entire game of Civilization around the tremendous growth bonus from the "We Love the President Day." Such a game generally consists of the following stages:
1) Rapid exploration and expansion (founding of the maximum defensible/bearable number of cities) -- in other words, the P.D.S. is unfortunately strongest in combination with a mild Infinite City Sleaze strategy. However, the final aim is LARGE cities, so each city is allowed space to reach near-maximum size, unlike in an extreme ICS strategy.
a) Heavy economic/agricultural development of the area around cities, at the expense of industrial development, and construction of contentedness, and later happiness/economy, infrastructure inside of cities, at the expense of science infrastructure, making up for this by diverting maximum national resources towards science in the period before economic infrastructure is built, and by moving to phase 3) as soon as possible. Also, establishing (late in this phase) as many trade routes as is feasible, with the highest feasible quality.
b) Meanwhile, maximizing security through wise diplomacy and by eliminating or pacifying land-based neighbors if possible, developing a complete road net (necessary anyway for part a) of this step), learning and using strong defensive techs, and building "counterattackers" and espionage units.
c) Meanwhile, building as many happiness/contentedness Wonders (special projects that each can only be built once, i.e. the first player to complete them shuts out the other players) as feasible.
3) This is the core step of the strategy. Discovering the republic or (preferred) democracy government types and implementing them as soon as step 2 is out of low gear. UNDER THE NEW GOVERNMENT, ALL RESEARCH STOPS (this is why no science facilities were built in step 2), and all cities' "gold" is divided between "luxuries" (which locally increase happiness and reduce unhappiness) and the national treasury. The treasury funds are in turn spent on contentedness/happiness/economy facilities. The luxuries percentage varies depending on the difficulty level and the player's preparedness; 30-40% is typical for the highest level.
The purpose of the luxuries and the expenditures on contentedness/happiness infrastructure should be clear: to qualify as many cities as possible, for the longest possible uninterrupted period, for the "We Love..." growth bonus.
Science is still trickling in during this phase due to diplomacy, espionage, and the side effect of trade caravans. (On the highest levels, the science stagnation in this period (and, to some extent, the preceding period) is actually a sneaky trick, as it extends the period in which you can gain new technologies "for free" from your neighbors.)
4) This phase continues until your economy is so strong that you can constantly buy relevant PD infrastructure and run a cash surplus. At this point, you start setting cities to build science facilities. Once a sufficiently large percentage of cities can complete libraries (normally or through rush-buying from your surplus) on a single turn, you do a mass rush-buy and change the division of your treasury funds so that science receives as much funds, or more, than your treasury.
By this point your (numerous) huge cities with strong internal and external infrastructure will more than make up for your previous science stagnation, and buying/building your way to previously-neglected industrial might will be a trifling matter. As far as defense goes, your economy will be so strong that, if nothing else, you'll be able to buy your way out of military crises. You can also, on the other hand, painlessly get aggressive and exploit the good public will that your strong economy has delivered (think "Operation Desert Storm" here) and become a devastating, high-tech "big stick."
What Have We Learned?
Many modern games (especially computer games) and their rulesets can be seen as parables. From Quake's "Kill 'em all" to Civilization's "You can win by brute force or by pleasing your people, but never without skill," these games deliver an intended or unintented message. These are especially vivid parables, because the player role-plays an important actor in them, and often feels rewarded for certain steps and penalized for certain other steps.
I've started using the term "ruleset parable" for precisely those parables that make use of rewards/punishments delivered through the ruleset. Civ-type games are full of ruleset parables, some good, some bad, some intended, some probably less so. The President's-Day parable is an especially strong one, because it is non-obvious, involves a lot of work, and is rewarded both through a tremendous improvement of the player's game state and some of the prettiest graphics and music in Civ/CivII. (For example, in Civ II the most famous portion of the Ode to Joy plays whenever a player's city begins celebrating.) I can almost unreservedly call the strategy "morally" pleasing: it rewards democracy, rewards a mixed economy (though it leans perhaps a little too much in favor of socialism, and for the most part rewards a peaceful approach.
I'm pleased that Sid Meier worked so hard to make a morally-pleasing strategy pleasing in these two powerful other ways as well. Skilled synergistic integration of various factors of Sid Meier's better games, and the interplay that makes the PDS possible and desirable could perhaps be called a "masterpiece within a masterpiece." It is an object lesson in synergy for future game designers.