The NIP-52 defines a new parameterized event kind, 31925, which represents a collection of calendar events. The format uses a custom replaceable list of kind 31925 with a list of tags as described in the text. The required tags are "a" reference tag to kind 31922 or 31923 calendar event being responded to per NIP-33, and "d" calendar name. Optional tags include "L" label namespace of freebusy per NIP-32, and "l" label of free or busy under the label namespace of freebusy per NIP-32. This new parameterized event kind can be useful for managing multiple calendar events at once.

Link: https://github.com/nostr-protocol/nips/blob/master/52.md