Answers

In order for shingles to sit and form properly, it should be at least 36 degrees fahrenheit (2.2 celsius) outside when laying shingles. 

Between 40 and 85 degrees is ideal!

I would recommend not less than 40 degrees. If the shingles are a dark color in direct sunlight they will seal in one day from the solar radiance from the sun even if temps are low. 

Each shingle manufacturer will specify if they have minimum temperatures for install.  40 degrees is a good rule of thumb. Below that, crews may have issues with their equipment (air compressors and hoses) working properly and shingles may not seal, espescially on northern facing slopes.

It is difficult to suggest an exact temperature for shingle installation. There are several factors involved such as your geographical location, the amount of sun exposure at your particular home, etc. As always, we would suggest referring to your shingle manufacturer's guidelines.