I want to take two days off from work, but I want to work over the weekend to compensate for the missing days. So I have to send an email to my client and I don't know what the correct expression for this is.
It would depend on how formal the situation is.
If dealing with a company or a new client you don't want to sound too casual as they might be concerned about delays and you may come across as unprofessional. I would give them a list of dates when you are available for work so they can see that you will be working at the weekend. Or you could say "Unfortunately I will not be available on [days] but I will be able to give this my full attention at the weekend."
In a more informal situation, with a client who knows and trusts you, there are a number of expressions you could use:
I'll make up the extra time/missing hours at the weekend.
I'll catch up at the weekend.
I'm free to work on this later in the week.