Names of people, places and organisations are called proper nouns. We spell proper nouns with a capital letter:
Mohammed Ali; Birmingham; China; Oxford University, the United Nations
We use capital letters for festivals:
Christmas; Deepawali; Easter; Ramadan; Thanksgiving
We use a capital letter for someone’s title:
I was talking to Doctor Wilson recently.
Everything depends on President Obama.
When we give the names of books, films, plays and paintings we use capital letters for the nouns, adjectives and verbs in the name:
I have been reading ‘The Old Man and the Sea’.
Beatrix Potter wrote ‘The Tale of Peter Rabbit’
You can see the Mona Lisa in the Louvre.
Sometimes we use a person’s name to refer to something they have created:
Recently a Van Gogh was sold for fifteen million dollars.
We were listening to Mozart.
I’m reading an Agatha Christie.