As the proposition it is either day or it is night is of great importance for the disjunctive argument, but for the conjunctive is of no value, so in a symposium (entertainment) to select the larger share is of great value for the body, but for the maintenance of the social feeling is worth nothing. When then you are eating with another, remember to look not only to the value for the body of the things set before you, but also to the value of the behavior toward the host which ought to be observed.
I had to look up the difference between disjunctive and conjunctive. Although understanding the opening sentence is not particularly important to understanding the overall meaning, it's still nice to know. Loosely, disjunctive arguments are "or" arguments: "A or B. Not A. Therefore B". Conjunctive arguments are "and" arguments, requiring both parts to be true for the whole statement to be true. Whether it is day or night could be useful in an "or" argument, but is useless in an "and" argument because it cannot be both day and night.
Looking at the statement with that in mind, it's saying something like: certain things are useful in one context, but not in another. Although greed can be satiating for the body, it can cause offence.
The last line, helpfully, restates the message clearly. I do wonder why Epictetus didn't limit this paragraph to the last line, it would have saved some confustion.