The following Feature request that was raised over 9 years ago in the forums , we'd like to see the same thing...

https://forums.manageengine.com/topic/preventive-maintenance-task-schedule-update-to-allow-%22every-2nd-xyz-day%22 

I would reply to the above ticket, but to be honest, the formatting of the manageengine forums website is terrible! - the bar hides all the buttons to post a new topic. :\

Anyhow, just in case you can't click on the above link, here's an extract of the request that was raised over 9 years ago, and is still required:

We perform our OS updates on certain sets of machines using a schedule of every 2nd Friday of the month. It would be very convenient to create Preventive Maintenance Task with this type of schedule.

We have strict deadlines for various repetitive tasks that must happen, and currently Service Desk Plus often raises the requests on a weekend because of the lack of scheduling options available in the Service Desk.

Thanks, 

John