One thing that advisors I coach frequently hear me say is: If you can't remember what you had for lunch four days ago, your clients can't remember what you told them casually in your last conversation (like "Please think of us next time someone may need a second opinion or a financial advisor").
The human mind works best with reinforcements, reminders, and more touches.
Think about Apple, the world's most recognized brands. We all know Apple. Then why does the company spend billions a year on advertising and brand awareness?
Which brings me to why I don't believe doing a once a year client appreciation event or a standard "lunch and learn" (I despise that term, by the way!), work. Even if the event is being sponsored by another firm, I see advisors spend multiple hours planning this one-time per year event, only for mediocre results.
Why are the results mediocre? You guessed it! Just like the lunch you had four days ago, pretty soon, all your hard work will be forgotten. If your goal is to enhance your overall client experience, gathering them once a year will be only as memorable as the last party they went to, even if everyone had an amazing time. If your goal is for them to invite their friends, so you can meet more of their social circle, again, your probabilities are quite low if it's just once a year.
So if you want to improve your firm's overall client experience, or to attract more prospective clients to your firm, convert your events into a series format.