Q. In the following sentence, would you stet the comma before “then”? “I’ll just come at the end of May, then.”
A. When then is used as an adverb to mean “in that case,” a comma can be helpful but isn’t strictly required. If you prefer close punctuation (i.e., a liberal use of commas), then you may want to retain such a comma. If you prefer an open style (with fewer commas), you can usually leave it out. (See also CMOS 6.16.) Whatever your style, you should retain a comma whenever then might be misread as “at that time.”
This use of then is analogous to the use of too and either in the sense of also (see CMOS 6.56 and “Commas with ‘Too’ and ‘Either’ ” at CMOS Shop Talk). Some writers always add a comma before that kind of too or either (or two commas in the middle of a sentence); others use a comma only rarely, as when ambiguity threatens (unlikely with too or either) or to add emphasis.
Either approach, consistently and thoughtfully applied, can work well. Alrighty then, on to the next question.