How to Create Recurring Series of Tasks in Lightning Experience and Salesforce Classic
Salesforce Lightning Experience:Enable Recurring Tasks in Lightning Experience and the Salesforce Mobile App
Steps to enable in Lightning Experience,
1) From Setup, enter Activity Settings in the Quick Find box, then select Activity Settings.
2) Select Enable Creation of Recurring Tasks.
3) Click Submit.
4) Add the Create Recurring Series of Tasks field to the Task Detail section of your org’s Task page layouts.
Use Case by Howard Zager – https://success.salesforce.com/answers?id=9063A000000pZoxQAE (Monthly recurrence for Tasks)