In April 1977, a group of disabled activists occupied a government building in San Francisco for nearly a month. The protesters were demanding the signing of Section 504 of the Rehabilitation Act, protecting disabled people from discrimination - it would be a breakthrough for the disability rights movement. Judith Heumann was one of the leaders of the sit-in.

Image copyright: Disability Rights Education & Defense Fund

In April 1969 Sikh bus drivers and conductors in the northern English town of Wolverhampton won the right to wear a turban on duty after a two year campaign. One of the key tenents of the Sikh religion is that men must grow a beard and long hair secured by a comb and covered by a turban. Farhana Haider has been speaking to Avtar Singh Azad who led the campaign in the fight for Sikh religious rights.

Photo Sikh bus driver 1972. Credit BBC

Tens of thousands of Polish officers were secretly executed in the USSR during World War 2. The German occupying forces reported the first mass grave, in the village of Katyn in 1943, but Moscow only admitted to the killings in 1990. Dina Newman speaks to the son of one of the murdered officers, Waclaw Gasiorowski. Photo: Gasiorowski family in Warsaw in 1936. Credit: family archive.

In the 1970s up to half a million people were killed during the brutal campaign of repression launched by Ethiopia's military regime called the Derg. Hear from one survivor who was imprisoned and tortured.

Photo: Human remains. Copyright: BBC.

Israa Abd El Fattah was one of the first Egyptian activists to use social media to help organise anti-government demonstrations. In April 2008 she tried to organise a general strike in protest at low wages, and rising prices. She was given the nickname "Facebook Girl". She says the experience of using Facebook to spread the word helped activists learn how to mobilise people before the Egyptian Uprising in the spring of 2011.

Photo: Israa Abd El Fattah in her office in Cairo in 2011. Credit: Khaled Desouki/AFP/Getty Images

America declared war on Germany on 6 April 1917, tipping the balance in favour of Britain, France and their Allies. The USA had resisted getting embroiled in the war in Europe for almost three years, but after the declaration of war, it sent troops to fight on the Western Front in France and Belgium.

Photo: Postcard of Joseph "Black Jack" Pershing (Centre - R), the US Army General who led the American Expeditionary Force in World War I, being welcomed in Boulogne, northern France, by French General Peltier . Credit: AFP PHOTO / Historial de Péronne

In the mid-80s, the world was terrified by HIV Aids caused by a lack of understanding and misinformation. In April 1987, Princess Diana opened the UK's first purpose built HIV Aids unit at London Middlesex Hospital that exclusively cared for patients infected with the virus. In front of the world's media, without wearing gloves, Princess Diana shook the hand of a man suffering with the illness. This gesture publicly challenged the notion that HIV Aids was passed from person to person by touch. John O'Reilly was a nurse on the ward at the time of the Princess of Wales' visit. He spoke to Farhana Haider about the landmark moment in the fight against HIV Aids.

(Photo: Princess Diana with an AIDS patient at the Middlesex Hospital April 1987. Credit REX/Shutterstock)

In April 1993, the Azerbaijani town of Kalbajar fell to ethnic Armenian separatists during the war over the disputed enclave of Nagorno-Karabakh. Photojournalist Khalid Asgarov tells Louise Hidalgo how he and his father were among a column of refugees who fled to safety on a two-day trek over the mountains.

Picture: A refugee woman from Kalbajar comforts two of her children after escaping over the mountains in Azerbaijan, 10th April 1993. (Credit: Dima Korotayev/AFP/Getty Images)

In April 1982 the film star Jane Fonda launched her first workout video - encouraging millions of women to "go for the burn". Hear how the idea of home workouts took off, and why she felt such a compulsion to exercise.

Photo: Jane Fonda on the red carpet for the Annual Academy Awards 2013. Credit: Frederic J. Brown/AFP/Getty Images

In March 2002 a massive ice shelf with a surface area of more than 3,200 square kilometres collapsed into the ocean around western Antarctica. The Larsen B ice shelf had existed for more than 10,000 years, but it split apart in a period of just 35 days. Mike Lanchin hears from the leading glaciologist Pedro Svarka who saw it happen.

Photo: Satellite images showing the collapse of the Larsen B ice shelf in early 2002 (Science Photo Library)