I am in a "transition period" shortly after having finished my MSc, and shortly before beginning my PhD. In the summer period the academic life doesn't cease, though, and sometimes an affiliation is asked for at scientific events. I am going to a conference soon, and indeed the organisers need an affiliation.
Is there some rule saying what affiliation I should mention in a situation like that: the past one, the future one, both, or neither?
Be pragmatic. If you need to provide affiliation for, for example, presenting work you have already done, then use the old affiliation. It would be odd to provide your coming affiliation in such cases. Even later when you are at your new place you may want to use the older affiliation along with your new one to show where you did the work and where you are now. If you need to provide contact information not related to work you have done then the new affiliation migt be good. That said, however, it would be wrong to use the new affiliation until you become officially affiliated by a contract (equivalent). From this perspective your old affiliation can be used until the new one is official (find out by asking your new location when can be applied). There is of course not wrong to clearly state that you will be at your new affiliation starting such-and-such time.
Maybe I'm a little too practical, but I would go with whichever email address will you be using most consistently in the future. Presumably that's the PhD one, assuming you have access to that email address. If anyone wants to contact you in six months that will be the more correct affiliation.
