I exclusively use a digital calendar. I use my Google calendar which I reference on my Smartphone through a widget and the calendar app. I set reminders, track appointments, color code certain commitments, etc. The hard part is being consistent about entering in the information. I like it because I can determine how often to receive reminders and I can also included notes about the appointment and the address or location. It’s a great way for me to keep track. But, it requires consistency.
My providers know that I’m in the habit of entering in appointments at the end of our sessions. I do still ask for physical cards as well as reminders since that helps me too to have it in my wallet as a backup. More for my anxiety than anything else.