Notification is required only for events exceeding New York State's non-residential social gathering limits of 100 people indoors or 200 people outdoors.
PLEASE NOTE: Limits increasing to 250 people indoors on May 19, and 500 outdoors on May 10.
These are non-residential events (gatherings outside of a household) that involve the preparation and service of food and beverages. Examples include wedding receptions, meetings, conferences, and other private venue events by invitation, not open to the general public. The Health Department must be notified at least 5 days in advance of the event.

The current limits on numbers of attendees:

Indoors Up to 100 people without testing or 101-150 people with testing or proof of vaccination; no more than 75% of capacity.
Starting May 19: Up to 250 people without testing or >250 with testing or proof of vaccination; capacity limited by required social distance of 6 feet.
Outdoors Up to 200 people without testing or 201-500 people with testing or proof of vaccination; no more than 75% of capacity.
Starting May 10: Up to 500 people without testing or >500 with testing or proof of vaccination; capacity limited by required social distance of 6 feet.