Yeeees... working days are working days, so half term (for example) wouldn't count and you would extend plans, timeframes etc accordingly. However, hopefully your policy also allows for flexibility so that if you dismiss someone near the end of the summer term, you don't have to give them two months to lodge their appeal.... there should not be unreasonable delays.

Presuming this is a member of staff who works term time only, then their time to submit an appeal must reflect when they would be expected to be at work. While it may drag things out, if you operate the other way round (as an enthusiastic Chair once agreed with someone without checking with me first) then you need to run procedures during school holidays when no one else is around to participate. Trust me, that was worse!