It's best to look at this as a three-stage process:

1. Activate the pass with the date you first plan to travel. You can deactivate the pass at any time before the first date of travel (if you change your mind about the start date).

2. Plan your trips by adding them to "My Trip". You can add as many journeys as you want, including multiple and/or overlapping journeys for the same day, if you haven't made up your mind. This step does not have any impact on the validity of the pass, so you can add journeys for a day even if you are not sure yet that you want to travel on that day.

3. When you are about to start travelling, go to "My Pass" and add a journey from "My Trip". At this point you will be asked if you want to use a travel day. Even then, if the travel day has not started, you can change your mind. (See: "Can I switch or cancel a travel day?" in the FAQ).

So in your case, where you have a flexi-pass valid for x travel days in one month:
a) the activation date sets the start of the one-month period (and can be changed any time before the date you first set).
b) the travel day is a day you want to travel on within this one-month period (and can be changed any time before the start of the day you choose).

Clearly, your first travel day should be the same as the activation date to get maximum benefit of the one-month overall validity period.