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Addressing the crowd, Benedict XVI said he was delighted by the greeting and referred to Germany as his "beloved homeland".

The World Youth Day festival, invented by the late Pope, is held in a different part of the world every three years.

Thousands of young Catholics from around the world have gathered in Cologne for a week of festivities.

http://news.bbc.co.uk/2/hi/europe/4161706.stm
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I would say that I go to Germany/France/Spain every second/third year.

I wouldn't sayt it is corect to write that I go to Germany/France/Spain every second/third years.

Would you write years?
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It's every second or third year.
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RotterThe World Youth Day festival, invented by the late Pope, is held in a different part of the world every three years.

Just to clarify: 'every three years' is also fine, in the example above.

'Every second or third year': each of those years is mentioned separately (= 'every second year or third year').

'Every three years': the three years are treated as a whole (= 'every period of three years').

MrP