I have taken this excerpt from a blog, The Days of Hajj Series by Yaser Birjas. It is well-written and simple to understand, even for a beginner and if one needs to refresh knowledge about the Hajj.
The Days of Hajj Series: Part 1 | Part 2 | Part 3 | Part 4 | Part 5
The Messenger of Allah (salla Allahu alayhi wa sallam) said: “ الحج عرفة “ or “Hajj is Arafah“.
The Arabic text implies a very important concept of the meaning of
Hajj. This short statement means that the whole hajj and its validity is
confined in the actual day of Arafah i.e. Hajj becomes valid only when the day of Arafah
is observed and witnessed by those who undertake the journey, the
pilgrims. Regardless how long the hujjaj stay in Makkah, if they missed
that single day, they have then missed the whole Hajj.