The "wait for X days then send" feature means that after a specified number of days, the email will be sent.
This includes non-scheduled days (e.g. weekends or days that are not on your schedule for sending).
For example, if you set a gap of 2 days, the follow-up will be sent 2 days after step 1 was initially sent. However, it will only be sent on the next active scheduled day.
If your schedule only allows sending emails from Tuesday to Thursday and you have a 2-day gap, if step 1 is sent on Thursday, the next email will be sent on the following Tuesday, as that is the next active scheduled day.