“season.” There is a season, or proper time, for everything. Knowing that can help us act in a proper and godly manner in life. For example, while there is usually wisdom and value to thinking things through and considering options, there are also times to act quickly; times when indecision or procrastination can be harmful or even deadly. Furthermore, some things in life, such as killing, can be very difficult in some circumstances, but there are times for that too (Ecc. 3:3). Life has times of great happiness and times of difficulty and sadness, but God has enabled us to go through all life has to offer and live in a godly manner.
The Hebrew could be translated “appointed time” (NAB, NASB, NET), but in English, the word “season” can have that implication. God does do some things at His appointed time, but there are also times when things happen due to cause and effect and people’s freewill action and God’s reaction. For example, when king Saul sinned and ignored God’s warnings, God took the kingdom from him. God’s move was not due to His predestining Saul to fail, but He did decide when and how to take the kingdom from Saul because Saul kept sinning.
Translating Ecclesiastes 3:1 with the phrase “appointed time” opens the door to the huge misunderstanding that everything happens “when it is supposed to happen.” That is not accurate. Lots of things that occur on earth were never God’s will or intention. For example, God does not want anyone to reject Him and die unsaved, but that happens all the time.
“under heaven.” The list that follows is not prescriptive, but descriptive. It speaks of the things that go on under heaven, in the realm of humankind.