There’s a new optimization option in Ads Manager for Reminders Ads using Instagram placements…
Using the Engagement objectiveWhen you create a campaign, one of the first things you'll do is select an objective. The campaign objective is your ultimate goal. Your selection will impact options, including optimization and delivery. Options include Awareness, Traffic, Engagement, Leads, App Promotion, and Sales. More, select “On Your Ad” as the conversion location.

Click the Engagement Type dropdown menu and select “Reminders Set.”

The performance goalThe Performance Goal is chosen within the ad set and determines optimization and delivery. How you optimize impacts who sees your ad. Meta will show your ad to people most likely to perform your desired action. More will be “Maximize reminders set” and it cannot be changed.

The only placements that will apply to Reminders are Instagram feed, Instagram profile feed, and Instagram Stories. Why? Reminders is an Instagram feature.

When you create your ad, the call to actionA call-to-action is a button or link on your ad that suggests the action you want your audience to take. Examples: "Learn More" or "Sign Up." More will be “Remind Me.”

Enter your Event Name and select both an event date and time.

Reminders are for online or offline events. We aren’t creating the event itself, but merely providing a way for people to get reminders about that event. It could be a product launch, Instagram Live, or store sale. This could be good for Black Friday sales.
You’ll need to provide details in the ad about what the event actually is that they are getting reminded about.
If you have a strong Instagram presence, this is something to try out. Let me know what you think!