I mean, we have the Spells8 calendars that you can add to your phone. They are updated by the team here and as long as you have your calendar notifications turned on you’ll get notified 
Here’s the community calendar with the challenges, group rituals, and other coven stuff.
→ Forum Community Calendar!
Here is the link to the 2024 Pagan Calendar collection that has many different types of calendars you can add to your phone!
→ 2024 Wiccan & Pagan Holiday Calendar – Spells8
As for other apps, I’m not sure if there’s such a thing. I’d be curious to know, too!