Please I need assistant, I am new to this site and struggling with a serious challenge.

 I work on a project where we trying to de-congest the ARVs medication collection process.

Every day we receive many people who come to collect medication. We are struggling with estimating when they should come for their next collection which is after three months. Taking into account that fact that public holidays and Sunday the facility is closed. (we give them medication for every day included Sunday, holidays but they cannot collect on these days)

Example For the month of June 2018

 Start date 2018-06-01     To 2018-09-01 (3 months period) Come 3days before expiration)

Problem: I want to calculate next collection date excluding Sundays and Public holidays only for the entire month of June 2018 (making sure the date does not fall on a Sunday or on a public holidays?

To ensure they come 3 days before their medication finishes.  If you collect 1^st June when is next collection and apply to the 30^th of June

I have a list of public holidays for 2018.

I was using the workday and networkday int

 Thank you