Send email notification 5 business days before activity set triggers on employee start date.

Hello Everyone,

I am working on lifecycle event onboarding for new hire. I have created lifecycle event names onboarding and add activity set to it names Prepare, Day 1, Week 1 and so on. Prepare activity set triggers immediately when the HR case is created and Day 1 activity set triggers when it matches the employee start date mention in HR profile form. I have to send an email notification to manager 5 business days before day 1 activity set is triggered in the lifecycle event. 
Please help me the ways to set "when to send" condition while creating a notification so that emails triggers on correct time. 
Note: Table used while creating notification is sn_hr_le_case