Cat going in for surgery on Tuesday but when should I take action earlier?

I recently took my 6 year old cat to the animal hospital yesterday. Unfortunately, they found cancer on his intestine (ileum). They scheduled a surgery Tuesday morning to remove it. However recently over the past week or so he has been eating less. I am able to entice him to eat a bit a couple times a day but not much. I’m fairly certain he is drinking water and using the liter box normally still but I’m still concerned. They took a blood test yesterday and all of his levels were good so they were happy about that and said his liver and kidneys appeared to be okay.

I know the mass in his abdomen is the reason he’s eating less. I just want to make sure he’s okay until Tuesday. How can I be sure he will be and when should I take him in sooner?