She has to tell her boss when she will be on holydays or when she is on holydays
I think the first one is better because you can ask the question she has to tell what not when
She has to tell her boss when she will be on holydays or when she is on holydays
I think the first one is better because you can ask the question she has to tell what not when
It is more clear that the trip is in the future if you say: She has to tell her boss when she will be on holiday.
Also correct options: She has to tell her boss when she is going on holiday. She has to tell her boss when she is going to be on holiday.