Oh come on... You just rotated this question 'What would my Non Muslim friends feel if I don't celebrate their festivals? Isn't that bad?'
'Is there any possibility that we can celebrate their festivals so that we don't hurt their feelings?'
There are many a places in the Qur'an which say that we should not make even friendship with the non Muslims (Read more here). And this question is about celebrating their festivals?.
The thing is Qur'an and Hadith want us to behave in a neutral way with them i.e. don't incline towards them, nor misbehave with them but to treat them justly.
So, in our case, staying neutral means not celebrating their festivals thereby not encouraging them, and at the same time we should not stop them from celebrating them. They have their rights (I'm talking about in a non-Islamic country especially).
About maintaining harmony?
I don't understand.... By just simply refusing to celebrate their festivals in a polite way, how is it at all considered 'not maintaining harmony'?
And acceptance of 'Holy days' or is it 'Holidays'? It would be cool to accept them as Holidays rather than 'Holy days'. By simply recognizing them as '
Holy days' Holidays doesn't mean we have to recognize them as 'Holy Days' too. As and always, we recognize them as Holy days just because we respect others and their feelings. That's it. That by no means, mean that we are supposed to celebrate those festivals along with the Non Muslims.
And "Isn't that bad?". Yes, it should be bad. Because at least by that refusal which they get, they will someday recognize what they were doing was wrong or they had/have to upgrade (in case of legitimate festivals like Christmas, although it is legitimate, there are no rulings in Islam that we recognize and celebrate it. Although there are places like the day of Ashura where Prophet (pbuh) finally said it is optional to fast on that day as our ancestors 'Jews' did because it was a happy day for them).
May the creator guide us all.