How about any location between 30 and 60 degrees latitude North and South?
Temperate is a subjective term. It can also have disparate meanings. Some people have bad tempers. Some claviers on the other hand are well tempered.
Climatologists in the old days defined three climate zones on the basis of Latitude and therefore of the solar incidence angle. The Tropical Zone on either side of the Equator spanned form 30 North to 30 South. The Temperate Zone from +/- 30 to 60 degrees; and the Polar Zone above +/- 60.
Modern technology and the availability of detailed information changed this simplistic approach. The Kerguelen Archipelago to cite just one of numerous example well within the Temperate Zone has a polar climate. Also think of Nepal at the limit of the Tropical Zone.
But not only altitude has to be taken into consideration these days systematising climate. Ocean current can bring freezing water to New York (the Labrador Current) or warm water to the Norwegian Coast (the Gulf Stream). Winds (dominant winds such as Monsoons) are relevant climate agents and so is nowadays humankind.
Therefore taking into account my original statement (temperate is a subjective term) I suggest to plan your holyday ignoring the chart I reproduced (from the net) here above. Check the innumerable weather sites instead for the place and date of your choice bearing in mind that Poland (Temperate Zone) can be freezing in the winter; and that you can merrily swim in the Canadian Artic in July; if you can bear the mosquitoes.