How it works

When you arrive at your town, the ground will be mostly covered with grass (unless, of course, it's wintertime), with a few patches of dirt. If you walk over the same area frequently, it eventually wears away. This wear will become more and more visible each time you play the game. There are three kinds of 'grass patterns': squares, triangles and circles. You can not change your 'grass pattern' (although it is possible to change it by hacking).

Seasons

There are 4 seasons in Animal Crossing. Every season your grass will change in another color. The following image is all four grass seasons/colors. On the picture you see 'square grass'.

Grass

A little grass grows back at 6am, but if you walk/run over a certain area enough a trail will begin to form. It takes about a fortnight (14 days) before a faint path becomes visible, though some players experience it sooner. It goes from barely noticeable footprints, to a clear trail after a while. It does not matter if you walk, jog or run on grass. It will experience the same wear regardless.

Grass needs to be present in order for five bugs to appear - the bell cricket, cricket, grasshopper, long locust, and migratory locust. These appear at various times of the year, but they all appear in September so it may be a good idea to designate a "nature reserve" area in town that is almost never walked on - even to dig up fossils or pull weeds, which should probably be done weekly in that area at most.

There are various patterns to grass, such as circles, stars, squares, and triangles. Each individual town will have a randomly-generated and permanent grass shape. This shape changes during winter, when the snow falls, then changes back again as the snow fades. e.g. If you have the circle pattern on your grass, the snow will be stars.

Snow

In early December, a notice is posted on your town's bulletin board alerting you to a low pressure system's arrival on December 10th. Following a full day of snowy weather, any grass covered areas will be converted to snow.

Snow, like the underlying grass, is worn away as it is walked over. Snow wear is more visible than grass wear because of the stark contrast between snow and dirt. Cracks in the ground will also be more visible. In addition to this, you will find snowballs in your town--your only method of catching dung beetles. These snowballs can also be made into a Snowman to attain Snowman furniture. Snow only falls on previously grassy areas, and natural dirt patches will always be bare. On February 18th at 6:00 a.m., the trees in your town start to show some green. On February 25th, at 6:00 a.m., all of the snow in your town will turn green, the snow on any ledges (buildings) will disappear, the snow on the bus will disappear, and the weeds will turn green.

Solutions to excessive grass loss

Some players find that after several weeks of play, there is very little grass left. In the winter, complete erosion of the snow can prevent players from obtaining Snowman furniture. One drastic solution is to time travel forward using the Wii system clock, loading one day at a time. Simply jumping forward a few months has no effect on the grass. Your grass should be fully grown back from a complete desert in 6 months, although if the situation wasn't too bad, it'll take less time than that.

An example of wear on a place where players walk very often, a ramp.

Another method is to place pattern paths to all the most visited places in town, using them exclusively while the grass is in a state of recovery. Note that the patterns themselves do not have an effect on the grass in any way. They are merely a tool to help players develop paths only where they are desired. Early regrowth is fragile; walking over it even once will cause it to visibly disappear. For this reason, players who use this method may have to refrain from shaking trees or digging up fossils and gyroids until the grass reaches a certain stage of recovery. Grass grows faster under trees and flowers and, for a town that has not yet eroded to the point of desertification, planting trees and flowers in problem areas may be enough to reverse or halt the damage. However, the act of walking to the flowers to water them will erode the grass you walk across to reach the flowers. Placing the flowers along the pattern paths themselves is an option while the town is in the early stages of recovery. Natural dirt spots will eventually grow back, too.

Whether you're trying to prevent future desertification or are in the process of regrowing your grass, there are are a few things that should be avoided. Unnecessary reloading of the game makes your grass more vulnerable to new wear. Not reloading is not a complete solution, however, as the grass becomes susceptible to the wear again after 6 a.m.. When there are multiple players of the same game file, it may be difficult to avoid reloading. In those cases, pattern paths may be additionally beneficial, as they encourage different people to take the same routes through the town. You should only walk/run where you want a path to form.

Grass wear is actually affected by the current season. During winter (especially December), the grass will be at its worst, wearing away quickly. During spring and summer (especially June), the grass will wear down roughly half as fast.

Note that the grass in your town is arranged in invisible, pattern-sized blocks. When a block of grass is growing back from total dirt, you'll be able to see a small circle of grass, dead center in that block. More bits of grass will appear within the block as time goes on, or the grass will shrink away if it's walked on.

Another possible solution is to get Nintendo to fix this For example: signing a petition like this one here: http://www.petitiononline.com/accf1357/petition.html

Grass Wear/Regrowth Timetable

Time to Fully Regrow (Assuming the square is completely bare)
Square not containing a flower: 85 to 255 days (roughly 3 - 8.5 months)
Square containing a flower: 27 days to about a month and half

Time to Fully Wear Down (assuming the square is as full as it can be):
Winter: 23 days to 30 days
Spring: 36 days to 64 days

RUNNING WILL INSTANTLY KILL YOUR GRASS!!!!!

Keep in mind that grass regrowth is not affected by the season. Only how fast the grass wears away is affected. Also keep in mind that while flowers drastically increase the rate at which grass regrows in the square the flower is on, it does not affect how quickly the grass wears away.