When is the best time to plant garlic?

 Fall is the best time to plant garlic

This is a tricky plant to grow. The way our temperatures have changed in Missouri.Some winters have been mild. Some with lots of snow, We have had outdoor family gathering on Christmas and Thanksgiving a few times. 
So just picking a set time is no longer an option. We were in the 80's f yesterday and they say rest of the week will be hot,So I have to hold off on planting right now.

Garlic requires 
1.Full sun and good drainage. That is why my garlic failed this year. Too much rain and not enough sun.
2. 1-2inchesof water per week, stop watering when leaves turn yellow.
3.Phosphorus (P) and potassium (K) are important for health and growth.
4. Keep garlic well weeded, as it cannot stand much competition.

 Hard neck garlic should be planted in the fall, when soil temperatures are at 50f (10c) The months of October or November will depends on the temperature of your area. If it is really hot you should hold off on planting. Soft neck garlic can be planted in spring, but bulb size may be reduced. The goal is to get well established roots and as much leaf as possible. That is all we have control over.

If planted too early too much top growth may happen before winter, if planted to late there may not be enough root growth to survive winter. So you really have to know the weather forecast to make a good judgement. The plant will start growing again if temperatures warm up in the winter. The plant can handle stop and start. But if it dies back 2 or more times it will reduce the yield.

 Roots will grow as long as the soil is not frozen
 The tops will grow as long as temperatures are above 40f (4.5 c)

Garlic needs at least 40 days at temperatures below 40f (4.5 c) to make the bulb divide into separate cloves.

Garlic is day length  sensitive
Bulbs start to form and leaf growth stops when the day length reaches 13 hours and soil temperatures are over 60f (15c).  Both has to happen before bulbs form.
.Temperatures cause harvest time to be early or late even if the day length is at least 13 hours.

It can be harder to grow in colder northern  climates Because it takes longer to reach the right soil temperature. 

It is important to have larger plants before they reach the stage of bulb growth. More leaf and root growth, will make a larger bulb..

Temperatures above 91f (33c) will end the growth of the bulb, and trigger the plant to start drying of the plant. The bulb will double in size that last month of growth. So removing the scapes (central stem)  on hard neck garlic will help increase the bulb size at that point.  .

Note garlic should be stored at temperatures of 50-60F (10-15c) Do not refrigerate
Temperatures between 40-50f (4.5-10c)  will cause sprouting before you are read to plant.  .

Here is a good read for in depth information. 

Notes: Don't wait for garlic leaves to start to die back to check for maturity, start checking for mature cloves about late June

The amount of nitrogen (N), phosphoric acid (P) and potash (K) in 

 common manures

Manure	Chicken	Diary cow	Horse	Steer	Rabbit	Sheep
N-P-K	1.1 .80 .50	.25 .15 .25	.70 .30 .60	.70 .30 .40	2.4 1.4 .60	.70.30 .90