For someone totally ignorant as to the why's and when's of going there. Why is this place so sacred and why at this time of year do millions go there?
Also I seen on the news the Saudi head blaming the crowds for effectively being too fanatical and steaming into each other. Is this fair?
My heart goes out to the families, this is an absolutely awful tragedy.
I'll attempt to explain, with the caveat that i'm not a Muslim, and this is my own understanding, so please feel free to correct me if a Muslim/ someone more knowledgeable would like to step in.
Mecca, or more specifically, the Ka'baa is the most holy place in all Islam, its revered in much the same way as the site of the Temple (where the Dome of the Rock Mosque is now) in Jerusalem is revered by Jews. Whilst not getting into the religious history/context, it is considered by Muslims to be the very first place of worship dedicated solely to Allah ever built. When Muslims pray, they are required to face the Ka'baa, no matter their position geographically, meaning the Ka'baa is the focal point of all Muslim prayer on Earth. Thus you can start to see the significance of Mecca and the Ka'baa in Islam, it's the holiest of holies, and non-Muslims are forbidden from even setting foot in Mecca.
As for the reason Muslims must go there, and at this specific time of year - every healthy, able bodied Muslim is required to make hajj (consider it a great pilgrimage) and physically see the Ka'baa at least once in their lifetime, it is the 5th pillar, or tenet of Islam and is essentially the ultimate expression of Muslim faith. Muslims are required to go during a specific 5 days in the last month of the Islamic year in order to consider the hajj fully fulfilled - you may have noticed within your Facebook or social circles many Muslims saying "Eid Mubrak" which refers to a holy day which marks the end of the period of hajj.
So this creates the situation you see in Saudi Arabia - millions of Muslims descend upon Mecca to fulfill the Hajj within the specified timeframe. I don't know enough to comment on the Saudi explanation of the incident, but this isn't the first time hundreds of people have died during the Hajj, which speaks perhaps to bad management of the event by KSA. There are many events throughout the world which attract far larger audiences, without the casualty rate. Iran certainly seem to think KSA has badly managed this. Apparently there were two streams - one heading out of Jamarat and one heading in, and they collided, causing a stampede effect, with both streams pushing up at the back as the line slowed, causing those caught in the middle to be crushed to death.
If true, then it seems they have been poorly controlled and directed - which implicates that the Saudi management for the event is to blame as Iran are suggesting. On the other hand, Iran is a regional rival of Saudi Arabia and thus jump at any chance to make them look bad. I would hope there will be a review which should hopefully help prevent this type of thing happening again, on the evidence of other large religious gatherings, its totally preventable.