Question: The halfpenny was first issued on what day when the United Kingdom and the Republic of Ireland decimalised their currencies?

Context paragraphs:
Two pence (Irish coin): The two pence (2p) (Irish: "dhá phingin" ) coin was the third smallest denomination of the Irish pound.  It was first issued when the Irish currency was decimalised on Decimal Day, 15 February 1971.  The coin was minted until 2000.  It was the third of three new designs introduced all in bronze, the others being the halfpenny and penny.  All featured ornamental birds on the reverse.

Postage stamps of Ireland: The postage stamps of Ireland are issued by the postal operator of the independent Irish state.  Ireland was part of the United Kingdom of Great Britain and Ireland when the world's first postage stamps were issued in 1840.  These stamps, and all subsequent British issues, were used in Ireland until the new Irish Government assumed power in 1922.  Beginning on 17 February 1922, existing British stamps were overprinted with Irish text to provide some definitives until separate Irish issues became available.  Following the overprints, a regular series of definitive stamps was produced by the new Department of Posts and Telegraphs, using domestic designs.  These definitives were issued on 6 December 1922; the first was a 2d stamp, depicting a map of Ireland (including Northern Ireland, which remained a part of the United Kingdom).  Since then new images, and additional values as needed, have produced of nine definitive series of different designs.

Penny (Irish decimal coin): The decimal one penny (1p) (Irish: "pingin" ) coin was the second smallest denomination of the Irish pound.  It was first issued when the Irish currency was decimalised on Decimal Day, 15 February 1971.  It was the second of three new designs introduced all in bronze, the others being a half-penny and a two pence coin.  All featured ornamental birds designed by Irish artist Gabriel Hayes on the reverse.

Halfpenny (Irish decimal coin): The decimal halfpenny (⁄ p) (Irish: "leathphingin" ) coin was the smallest denomination of the Irish pound.  It was first issued when the Irish currency was decimalised on Decimal Day, 15 February 1971.  It was one of three new designs introduced all in bronze and featuring ornamental birds on the reverse.  The coin value was weakened by inflation and very few were produced beyond the initial run for 1971.  It was removed from circulation and demonetised on 1 January 1987.

Martin Coles Harman: Martin Coles Harman (born 1885 in Steyning, Sussex, died 1954) was an English businessman who bought the island of Lundy in 1925 and proclaimed himself King of Lundy.  He later issued an independent Lundy currency of Half Puffin & 1 Puffin coins, which were nominally equivalent to the British Halfpenny & Penny, resulting in his prosecution by the United Kingdom authorities for issuing illegal coinage under the Coinage Act of 1870.  The House of Lords found him guilty in 1931, and he was fined £5 with fifteen guineas expenses.  The coins were withdrawn and became collectors' items.  In 1928, Harman established the Lundy pony.

Decimal Day: On 15 February 1971, known as Decimal Day, the United Kingdom and the Republic of Ireland decimalised their currencies.

Polymer banknote: Polymer banknotes are banknotes made from a polymer such as biaxially oriented polypropylene (BOPP).  Such notes incorporate many security features not available in paper banknotes, including the use of metameric inks. Polymer banknotes last significantly longer than paper notes, causing a decrease in environmental impact and a reduced cost of production and replacement.  Modern polymer banknotes were first developed by the Reserve Bank of Australia (RBA), Commonwealth Scientific and Industrial Research Organisation (CSIRO) and The University of Melbourne.  They were first issued as currency in Australia during 1988 (coinciding with Australia's bicentennial year).  In 1996 Australia switched completely to polymer banknotes.  Other countries that have switched completely to polymer banknotes include; Brunei, Canada, New Zealand, Papua New Guinea, Romania, Kuwait and Vietnam.  The latest countries to introduce polymer banknotes into general circulation include: the United Kingdom of Great Britain and Northern Ireland, Nigeria, Cape Verde, Chile, The Gambia, Nicaragua, Trinidad and Tobago, Republic of Maldives, Israel and Nepal.

Three Halfpence Red: The Three Halfpence Red, first issued on 1 October 1870, was the first Three Halfpenny postage stamp issued in the United Kingdom.

Postage stamps and postal history of Bangladesh: Bangladesh first issued its own postage stamps upon gaining independence in 1971.  A set of eight stamps, with various motifs including a map of the country, were issued.  Shortly after, stamps in eight values were overprinted "Bangladesh Liberated" in both English and Bengali were prepared in the United Kingdom, but only three values were issued in Bangladesh.

Halfpenny Rose Red: The Halfpenny Rose Red, first issued on 1 October 1870, was the first halfpenny postage stamp issued in the United Kingdom.

Supporting paragraphs: Halfpenny (Irish decimal coin) Decimal Day
Answer: 15 February 1971