Is there any built-in functionality in django to call a method on a specific day of the month?
For your 1st question, i don't think there will be any issue if you're using celery and celery beat for scheduling this task. Assuming your production server has 2 cores (so 4 threads hopefully), you can configure your celery worker (not the beat scheduler) to run using 1 worker with 1/2 thread. At the 15th of a month, beat will see that a task is due and will call your celery worker to accomplish this task. While doing this your worker will be using 1 thread and the other threads will be open (so your server won't go down). There are different ways to configure your celery worker depending on your use case (e.g. using gevent rather than regular thread), but the basic config should be fine.
Well I think you should keep a column in your table to track which ones were successfully handled by your code, and which failed. Celery dashboards will only show if total work succeeded or not, and won't give any further insights.
Hope this helps!
Brief intro of the app: 

I'm working on MLM Webapp and want to make payment on every 15th and last day of every month.
Calculation effect for every user when a new user comes into the system.

What I did [ research ]

using django crontab extension
celery

Question is:
-- Concern about the database insertion/update query:

on the 15th-day hundreds of row generating with income calculation for users. so is there any better option to do that?
how to observe missed and failed query transaction?

Please guide me, how to do this with django, Thanks to everyone!