That meaning of "surgery" is uncountable, so don't use "a."

Both versions of the sentence are grammatically correct, but they have slightly different meanings. "I'm having a foot surgery tomorrow" suggests that you are having a specific surgery on your foot, perhaps one that you have previously discussed or scheduled with someone. "I'm having foot surgery tomorrow" suggests that you are having some type of surgery on your foot, but the specific details may not be as important or may not have been previously discussed. In general, if you are referring to a specific surgery, it is more common to use the article "a" before the noun. However, if you are talking about a general type of surgery, you can use the noun without an article.

With the word "operation", you would use an article: "I am having a foot operation". This doesn't work with "surgery". It is not interchangeable with "operation". "Surgery" is the name of the practice that is performed by surgeons. Surgeons study surgery in order to learn how to do operations. "I am having a foot surgery" is grammatical but doesn't make sense. On the other hand, "I am having surgery" does make sense. To understand better, consider a different situation. You may have "schooling" to study a language. Similarly, you may have surgery to have an operation. "Surgery" is more of a process, like "schooling", rather than a specific instance.

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