Mashhad is Iran’s holiest and second-largest city. Its raison d’être and main sight is the beautiful, massive and ever-growing Haram (shrine complex) commemorating the AD 818 martyrdom of Shiite Islam’s eighth Imam, Imam Reza. The pain of Imam Reza’s death is still felt very personally well over a millennium later and more than 20 million pilgrims converge here each year to pay their respects. Witnessing their tears is a moving experience, even if you’re not Muslim yourself.
Mashhad is also a good place to buy carpets, and it's a staging post for travel to Turkmenistan, Afghanistan and the little-touristed Khorasan region.
Be aware that during No Ruz and major Muslim holidays, almost all accommodation and transport will be booked out months in advance. (By contrast, at other times visiting Mashhad can prove quite a bargain.)