If you are taking 7 days unpaid leave, then your salary will be down that amount, relative of tax payments. Is this unpaid leave part of your entitlements - e.g. parental leave ?
Have you a payroll / HR team where you work? You will need to give them as much notice as you can in relation to your time off. The payroll dept will sort out everything in relation to reduced pay / tax that time. Usually, your manager would need to ok this time off, so again talk to them well in advance of taking this time. Please note, it could be refused first time around if it's not a good time in relation to deadlines / workload etc. It's uncommon to be refused, but not unheard of.