you are missing something, as countless people have explained. For many things, it is not enough notice. People have arranged plans, trips, courses, tours, events, etc that always take place on that weekend. Money has been paid, things have had long-standing arrangements etc. They can't all be moved to the Friday, and even if they were, it wouldn't necessarily be appropriate, as they might conflict with the VE celebrations.
Nobody is objecting to the day itself, or the need for commemorating it. These 'you should be grateful' arguments get tiresome, because it's like nobody can object to anything at all about the way something is done, if it's for a good cause. And we all know that's not true. What if you were told a couple of weeks in advance that you were going to have to give up something you'd planned, lose money, change everything around etc - you'd be a bit put out, even if it was for an exceptionally good cause. Imagine you were told that you couldn't possibly complain about that, because of the reason. You'd probably think that was unfair, and complain anyway. So it's not the case that because it's a good reason, that nobody can ever complain about the details of something. So we are not arguing about the principle of it; we are just disagreeing about the length of notice that people think is reasonable. People are expressing that for them, it is not enough time because of long-standing arrangements. Other people haven't realised that there are so many things that regularly happen that weekend, that have been organised, paid for, booked etc so far in advanced, and that actually it does matter to people, irrespective of it being for an excellent cause. Maybe they don't need to arrange things that far ahead, or maybe they aren't part of any groups or organisations that have long-standing commitments for that weekend, and for them, it is loads of notice, but that's not true for everyone.
People are more than happy to commemorate and be grateful for VE day, but they can still disagree about the way the bank holiday was arranged.