When whatever event triggers your code, I think what you are asking is to do something like
if (this.WindowState == FormWindowState.Minimized) this.WindowState = FormWindowState.Maximized;
So if your window is minimised, when the timer event triggers, your form will be maximised.
I would imagine you check for the date and time when the timer is triggered, then show the form if the date is the right one (rather than having a timer for each date you want to be reminded of) but you say you are a novice - so go for whatever you are comfortable with and grow from there.
timer.Stop will stop your timer.