We all know teaching operates 90% on goodwill, and like most teachers, it sounds like you would be more than happy to accommodate this change if you were able to.
Taken from NEU website, this is their stance:
All schools should have a calendar of the activities which make up teachers’ directed time – published in the summer term for the next academic year and consulted on with staff via trade unions. They should also give each teacher their own personal directed time calendar showing their individual commitments.
Schools should not allocate the maximum 1,265 hours to each teacher – they are advised to allow a contingency in order to cover unexpected additional events and commitments (though these should be rare and subject to consultation with staff).
NEU reps/members should ask their head or leadership team for the directed time calendars for both individuals and departments with the school’s calculation of directed time during the summer term term (if it has not been provided) and ensure staff are consulted before it is implemented.
I would start by having a reasonable conversation with your headteacher to see if you can mutually agree an alternative, and if not then it probably needs to be a conversation with your union. Teachers don't have the option of booking leave, and it is completely absurd to suggest that you can never book an event, concert, theatre, meal, night away, INCASE directed time is changed.
Hopefully your headteacher will be reasonable about it anyway and you won't need to push further. An option to avoid having parents back in would be to do it via teams/phone instead?