What can be reasons for minor pain in my upper left calf? you can see a visible green vein there. i'm going to the doctor on the 15th. can it wait?

Likely can wait: Varicose veins are very common and can be tender to touch. If it were to become swollen, red , and tender.... It would be more worrisome. If that were to occur you would want to get treatment sooner.

Probably minor.: Calf pain is always of concern because of the potential to be due to a clot (superficial or deep). What you are describing has none of the characteristics of a deep clot and, although you see a green vein, it also doesn't sound like a superficial clot. Other things can cause calf pain such as muscle pull, trauma or straining. If you have worsening pain then you should see your doctor sooner.