Alcohol slows your bodies ability to make platelets. Platelets usually drop during chemo treatments. Platelets are what clots your blood so if your platelets are low, you may bleed a little bit during surgery.
Also, you will be receiving many drugs that may not interact well with alcohol. Some doctors prefer that you stop early to make sure all the alcohol is out of your system.
The first time I had surgery and my platelets weren't an issue, I had a glass of wine the night before my surgery (clear liquid right!). My doctor did not put any restrictions on me. The second time I had surgery, my platelets were low so I had stopped drinking way before surgery but I still had a bleeding problem.
Good luck wth your surgery.