Does until include that day?

When until refers to a day or date, the meaning can be ambiguous because “until” refers to a particular point in that day or date rather than the whole day or date.

When people say “I’m out of office until Monday/May 2,” they generally mean that the state of “being out of office” will continue up to and end at the beginning of work, school, etc. on Monday (e.g., at 9 a.m. when they start work on Monday), so they will be back at work and available on Monday/May 2.

If they say “I’m here/available/working until Wednesday,” they generally mean that the state of “being here/available/working” will continue up to the end of work, school, etc. on Wednesday (e.g., 5 p.m. on Wednesday), so they will still be here/available/working on Wednesday.

To avoid confusion, you can specify the “end point” of the state or situation (e.g., “I’m out of office until 9 a.m. on May 2,” or “I’m available until 5 p.m. on Wednesday”).

Have you tried QuillBot’s free Grammar Checker? You can use it to check your out of office email messages.