Download Cron scheduler FREE Unity. 

This asset will allow you to schedule events using a cron based approach.

It contains an adaptation of the cron implementation by cravens that can be found, along with an in-depth explanation of the cron on the Random Sparks website. If you want more information about the cron syntax you can visit the Wikipedia page for cron

[I will try to resolve any doubt in the unity’s forum post for this plugin]