Question: What remembrance day held on April 25 features a riderless horse leading the march?

Context articles: Anzac Day
Remembrance day of the Latvian legionnaires
Remembrance Day bombing
Remembrance Day at the Gettysburg Battlefield
Yom HaShoah
Holocaust Memorial Day (UK)
Shrine of Remembrance
Riderless horse
Singaporean by-elections, 1961
Mullivaikkal Remembrance Day

Anzac Day is a national day of remembrance in Australia and New Zealand that broadly commemorates all Australians and New Zealanders "who served and died in all wars, conflicts, and peacekeeping operations" and "the contribution and suffering of all those who have served".
 Observed on 25 April each year, Anzac Day was originally to honour the members of the Australian and New Zealand Army Corps (ANZAC) who fought at Gallipoli against the Ottoman Empire during World War I.
Remembrance day of the Latvian legionnaires (Latvian: "Leģionāru piemiņas diena" ) or the Latvian Legion Day is a day on 16 March, when soldiers of the Latvian Legion, part of the Waffen-SS, are commemorated.
 It was made an official remembrance day in Latvia in 1998 but the Latvian government abolished the day as an official commemoration in 2000.
The Remembrance Day bombing (also known as the Enniskillen bombing or Poppy Day massacre) took place on 8 November 1987 in Enniskillen, County Fermanagh, Northern Ireland.
 A Provisional Irish Republican Army (PIRA) bomb exploded near the town's war memorial (cenotaph) during a Remembrance Sunday ceremony, which was being held to commemorate British military war dead.
 Eleven people (ten civilians and a police officer), many of them old age pensioners, were killed and 63 were injured.
 The IRA said it had made a mistake and that its target had been the British soldiers parading to the memorial.
Remembrance Day is an annual Gettysburg Battlefield event for the anniversary of the 1863 Consecration of the National Cemetery at Gettysburg during which President Abraham Lincoln delivered the Gettysburg Address.
 Initially commemorating the Grand Army of the Republic, the event is held on a weekend (e.g., Saturday November 16, 1957), and the Consecration anniversary was resolved "Dedication Day" by the U.S. Congress in 1949.
 A costume ball held the Friday evening before Remembrance Day, a "Remembrance Illumination" of Gettysburg National Cemetery graves (instead of on Memorial Day), a fundraiser at the Gettysburg Cyclorama, a Citizenship Ceremony, and other events promote tourism to Gettysburg, Pennsylvania.
Yom Hazikaron laShoah ve-laG'vurah (יום הזיכרון לשואה ולגבורה; "Holocaust and Heroism Remembrance Day"), known colloquially in Israel and abroad as Yom HaShoah (יום השואה) and in English as Holocaust Remembrance Day, or Holocaust Day, is observed as Israel's day of commemoration for the approximately six million Jews who perished in the Holocaust as a result of the actions carried out by Nazi Germany and its accessories, and for the Jewish resistance in that period.
 In Israel, it is a national memorial day.
 It was inaugurated in 1953, anchored by a law signed by the Prime Minister of Israel David Ben-Gurion and the President of Israel Yitzhak Ben-Zvi.
 It is held on the 27th of Nisan (April/May), unless the 27th would be adjacent to the Jewish Sabbath, in which case the date is shifted by a day.
Holocaust Memorial Day (HMD) (27 January) is a national commemoration day in the United Kingdom dedicated to the remembrance of those who suffered in The Holocaust, under Nazi Persecution, and in subsequent genocides in Cambodia, Rwanda, Bosnia and Darfur.
 It was first held in January 2001 and has been on the same date every year since.
 The chosen date is the anniversary of the liberation of Auschwitz concentration camp by the Soviet Union in 1945, the date also chosen for the International Holocaust Remembrance Day and some other national Holocaust Memorial Days.
The Shrine of Remembrance, located in Kings Domain on St Kilda Road, Melbourne, Australia was built as a memorial to the men and women of Victoria who served in World War I and is now a memorial to all Australians who have served in war.
 It is a site of annual observances of ANZAC Day (25 April) and Remembrance Day (11 November) and is one of the largest war memorials in Australia.
A riderless horse (which may be caparisoned in ornamental and protective coverings, having a detailed protocol of their own) is a single horse, without a rider, and with boots reversed in the stirrups, which sometimes accompanies a funeral procession.
 The horse follows the caisson carrying the casket.
 A riderless horse can also be featured in military parades to symbolize fallen soldiers.
 In Australia for example, it is traditional for a riderless horse known as the 'Lone Charger' to lead the annual Anzac Day marches.
There were 2 by elections held in 1961.
 The first by election was held on 29 April 1961 and the nomination day was held on 11 March 1961 while the second by election was held on 15 July 1961 with the nomination day held on 10 June 1961.
Mullivaikkal Remembrance Day (or simply Mullivaikkal Day; Tamil: முள்ளிவாய்க்கால் நினைவு நாள் "Muḷḷivāykkāl Niṉaivu Nāḷ") is a remembrance day observed by Sri Lankan Tamil people to remember those who died in the final stages of the Sri Lankan Civil War.
 It is held each year on 18 May, the date on which the civil war ended in 2009, and is named after Mullivaikkal, a village on the north-east coast of Sri Lanka which was the scene of the final battle of the civil war.