ADT 1

Yes it is mandatory 

I agree with Shailvi. It is not mandatory for appointment of First Auditor. However, it is always advisable to file the same.

Why it is advisable, Can U Tell us

and why it is not mandatory 

See Section 139(1) carefully 
Also See Rule 4 of the Companies  (Audit & Auditors) Rules,2014.

Notice of appointment of auditor is to be filed within 15 days of meeting in which auditor is appointed 

Form ADT-1 is requried to be filed pursuant to Section 139(1) of Companies Act, 2013. Section 139(1) only speaks about appointment of Auditor for five/three years. Appointment of First Auditor is as per Section 139(6). Rule 4 gives reference of only Section 139(1) and not Section 139(6).

By this way, many people is of opinion that it is not necessary to file ADT-1 for First Auditor. However, when you see it logically, it would be better to file the form since you are appointing an Auditor. Hence, it is advisable to file the same.

The first auditor will hold office till First AGM?