I have extensive experience with maintaining a calendar and setting reminders not only for myself but also for the clients.
As far as tasks, I like to use Tom's Planner. It is a project management software that allows me to input lists of different tasks that need to get done and be able to check them off as I go and a way for my clients to see what's being done. I like to put all work into Google Drive. I use GSuite for my business and it's probably the best tool I have to be able to share documents back and forth with my
So as far as managing a calendar, we can do it one of two ways. Since I basically use Google for everything, what I have done in the past is created a shared calendar between my clients and I to enter in meetings, travel, and dates when tasks are due. Then I would set notifications for the day before and 30 min - 1hr before the scheduled event.