Most of the answers to your questions can be found on the Steinhardt Graduation and NYU All-University Commencement websites.
But here are some highlights:
Graduation ceremonies are only held in May. If you are a September or January graduate, you are invited to participate in the May ceremonies. However, attendance at both ceremonies (Steinhardt Graduation and NYU Commencement) is optional.
If you plan to go to the graduation ceremonies (whether you are a September, January, or May graduate), you must apply for graduation.
To apply for graduation, you can do so in your Albert account.
If you haven't applied for graduation (within the application deadline), you must contact email Graduation Services at graduation@nyu.edu as soon as possible.
NYU Graduation at Yankee Stadium
For the date and time of the ceremony, check the Steinhardt Graduation website.
At this ceremony, each student will go across the stage to have their name announced as a graduate.
Visit the Steinhardt Graduation website for information on ticket distribution, dates, and information on requesting additional tickets. Updated information becomes available closer to the May graduation ceremonies.
Important Note: Because Graduation ceremonies are organized centrally by the university and school, your advisor does not know how many tickets you will receive, whether extras will become available, and/or how many extras you are able to request.
Commencement is typically held on a Wednesday in mid-May.
Information about obtaining appropriate academic attire can be found on this website.
Requesting tickets for the NYU Commencement is handled separately from requesting tickets for the Steinhardt Ceremony, including the Extra Ticket Policy. Updated information becomes available closer to the May graduation ceremonies.
Important Note: Because of the limited space at Yankee Stadium, students are only allowed 4 tickets in total (1 for the student, 3 for guests). There is NO guarantee that you will be able to receive additional guest tickets from the university. If you anticipate needing more guest tickets, pay attention to the information provided for requesting additional tickets.
The All-University Commencement is held at Yankee Stadium around mid-May, rain or shine.
As the date approaches, check the weather and be prepared for inclement weather.