The year. Otherwise your event for the 18th April might be in a few weeks, or it may be a stale webpage from your very successful and very over event in 2004.
The weekday. Otherwise I can’t intersect your event’s day and my (ridiculously complicated) personal calendar in my head and figure out if this is a Tuesday probably-I-could-swing-it event or a Wednesday nope-I’ll-be-in-Melbourne event without authenticating to some device and opening my calendar navigating to the day and… SQUIRREL.
If you are kind-hearted, you could add a few of very very many pieces of information suggested in the AdaCamp template lovingly brought to you by seven revisions of AdaCamps. But you can start off with the year and weekday.
Related: Terms not to use when negotiating meeting times, an incomplete list