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Biannual describes something that happens twice a year.
Biennial indicates an event occurs once every two years.
‘The biannual meetings between Jennifer and her ex, for the sake of the children, were too frequent for her peace of mind and she declared a wish to reduce them to just once a year in future.’
‘The road race for charity was held biennially, alternating with a sponsored silence for the children, so that money was raised each year for the good cause.’
15 May 1996 - Bill Clinton, US President, signed Megan's Law, ensuring the public would be notified if a dangerous sex offender was released into their community.