@Harshavardhan
I don’t think it’s possible to trigger a designer workflow on the 15th of every month by using an out-of-the-box way.
1.If you use SharePoint Server, you could first use PowerShell to run the workflow, then use windows task scheduler to run the PowerShell on the 15th of every month.
How to start workflow in SharePoint 2013 by using PowerShell
Windows Task Scheduler: Configure to run a PowerShell Script
2.If you use SharePoint Online, you could build a scheduled cloud flow to achieve this.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our [documentation][4] to enable e-mail notifications if you want to receive the related email notification for this thread.
2: https://social.technet.microsoft.com/wiki/contents/articles/38580.windows-task-scheduler-configure-to-run-a-powershell-script.aspx [4]: https://learn.microsoft.com/en-us/answers/articles/67444/email-notifications.html