Harvest is important because we all depend on crops for our food throughout the year.
The most important harvests are connected to the time when grains such as wheat and maize ripen. These cereals provide us with bread, pasta, breakfast cereals and other foods and they are an important part of our diet.
Harvesting the cereal crops was traditionally connected to a religious celebration called the Harvest Festival. The harvest festival is actually not an official day in the Christian calendar, but most churches and schools observe it anyway. But, as a result of this, it can be any day that the community chooses.
You get an idea of how important a harvest is by the fact that the first harvest of the Pilgrim Fathers in North America in 1621 was such an important step towards the colonists surviving in the new world that it is now marked by an official public Holiday - Thanksgiving Day.