noun-modifying (irregular)
quinquennial; recurring every five years
Used as a prenominal modifier in formal or administrative contexts, such as government plans or reports. Often appears in phrases like 五箇年計画 (five-year plan).
政府は新たな五箇年計画を発表した。
The government announced a new five-year plan.
五箇年ごとに大規模な調査が行われる。
A large-scale survey is conducted every five years.
Variant using numeral ５ instead of 五.
Numeral variant with katakana カ.
Numeral variant with small katakana ヵ.
Numeral variant with small katakana ヶ.
Numeral variant with hiragana か.
Numeral variant with full-width katakana ケ.
五年 simply means 'five years' as a duration or point in time, while 五箇年 emphasizes the recurring or planned interval, often used attributively.
Compound of 五 (five) + counter 箇年 (years, using the counter 箇/カ/ヶ). The counter 箇 is a variant of 個, used in formal counting. The reading ごかねん is standard.