Should I alter personal holidays for a research visit? The group of my professor and another professor from a distant Asian country have signed a cooperation agreement. 

As part of the cooperation, me as a rookie graduate student have to visit the other professor. Unfortunately, the proposed days of the visit interfere with a few days of a personal holiday. On some other time-frames it is either too late or the host professor has deadlines to work on.

The only time-frame which fits all is the one which infers with my plans.

Both the host professor and my professor in their email communication asked explicitly whether the proposed date fits me. Of course it doesn't, but I don't know if I should say that or should I simply sacrifice my plans (a weekend + two bank holidays), lose a few days of personal traveling, and opt for the visit?
How important is the research to you? And how hard is it for you to replan? 

An enthusiastic grad student probably would jump onto the opportunity, but if you see it as a chore, and you do not see direct repercussions (after all, they asked you whether it works), you might decline it.

Of course, you may still be enthusiastic, but it may mean losing out on a pre-booked, expensive trip to the outback/jungle/mountains/etc. (or something else that requires a lot of organisation and/or money to arrange). In this case, the decision will be less clear-cut.