April
This diversity calendar starts with month-long celebrations:

Arab-American Heritage Month

Autism Awareness Month

Earth Month

Tartan (Scottish-American) Heritage Month

April 2: World Autism Awareness Day, meant to raise awareness of the developmental disorder.
April 2-May 2: Ramadan, the ninth month of the Muslim year and a holy month celebrating when Mohammad received the revelations of the Quran, spent fasting, reflecting, and praying.
April 8: Yom Hashoah, or Holocaust Memorial Day, celebrated in Israel and around the world as a day of remembrance for the 6 million Jews that died in the Holocaust.
April 10: Ram Navami, a Hindu holiday celebrating the birthday of Lord Rama, the seventh avatar of Vishu.
April 10: Palm Sunday, a Christian holiday marking Jesus's entry into Jerusalem and the start of the Holy Week.
April 14: Holy Thursday, a Christian holiday commemorating the Last Supper between Jesus and the Apostles before his crucifixion.
April 14: Lord's Evening Meal, celebrated by Jehovah's Witnesses in memory the Last Supper celebrated on the first night of Passover in 33 CE.
April 14: Vaisakhi, the Sikh New Year and a celebration of the founding of the Sikh community in 1699
April 13-15: Songkran Festival, the Thai New Year, also celebrated as the Buddhist New Year.
April 15: Good Friday, a Christian holiday marking Jesus's crucifixion.
April 17: Easter, celebrated by Christians as the day Jesus rose from the dead after dying on the cross.
April 13: Equal Pay Day, at the time of writing; this day marking the pay gap between men and women moves depending on the actual pay gap.
April 14: Mahavir Jayanti, an important holiday celebrated by Jains commemorating the birth of Lord Mahavira.
April 15-April 23: Passover or Pesach, an eight-day Jewish festival celebrating when Israelites were freed from slavery in ancient Egypt.
April 21-May 2: Rivdan, a Baha'i festival celebrating when Baha'u'llah resided in paradise and proclaimed his mission as God's messenger.
April 22: Earth Day, promoting sustainability and environmental protection.
April 23: National Day of Silence, a protest against bullying and harassment of LGBTQIA+ individuals by students who take a vow of silence.
April 24: Armenian Martyrs' Day, honoring the 1.5 million Armenians killed by genocide in Turkey.
April 24: Pascha, Orthodox Easter.
April 29: Laylat al-Qadr, the holiest night of the year for Muslims, celebrated on the 27th day of Ramadan and commemorating the night that the Quran was revealed to Mohammad.