Botticelli's Mystic Nativity is one of the best known paintings in the National Gallery in London. One of the reasons it's called "Mystic" is because the painting is full of symbolism taken from the Book of Revelation. This is stated explicitly in a Greek inscription at the top of the picture, together with the date it was painted: the year 1500. Now 1500 may not be quite as round a number as 1000 or 2000, but it was round enough to attract its share of "End of the World" prophecies -- including the Second Coming of Christ, which is what Botticelli was alluding to in this painting.
As well as its religious symbolism, The Mystic Nativity is important for its "mystical" artistic style, which had a major influence on British painting of the Victorian period -- in particular that of the Pre-Raphaelites (in the year 1500 Raphael was 17 years old whereas Botticelli was 55 -- so he was literally a pre-Raphaelite!).
At this point it would be logical to include an image of The Mystic Nativity, but it's such a hackneyed Christmas image that I won't bother (there's a good zoomable version on the National Gallery website). Instead, here is a less well-known Nativity painting in the Pre-Raphaelite style. This is a huge watercolour by Sir Edward Burne-Jones called The Star of Bethlehem, which was painted in 1890 for the Birmingham Museum and Art Gallery. To me this captures the essence of the word "mysticism" much better than the Botticelli version.