If you would like to create a class which is free & included with all membership types, simply head to Manage > Classes > click the blue 'Add' button in the far right corner. Fill in the class information as normal (name, description, trainer, etc.), and under the 'Price / Eligibility' section, select 'Single Price' and set the price for 'Drop In Clients' and 'Members' to '0', as per the screenshot below.
It is also advisable to utilise the start & end date options (see here for more info) when setting up this class, if it is a one day event which does not recur the following week.
However, it is extremely important to note that, if any of your members have any booking credits on their account, the system will always deduct a credit in this case. One suggestion would be to go through the booking list for the free class after it has passed, and manually re-assign booking credits to members where applicable.
Alternatively, if you would prefer not to deduct a credit, simply apply the steps above to set up your free class as a course instead. This will not deduct a credit; just charge the price which is set.