So, I tried to upgrade from 6.3.1 to 6.5.5 a lot of fields are missing in the new database which were in the old database. And now when i go to the forum i get this error in postlist.php

Line#: 794
SQL Error: Unknown column 't1.B_CalDay' in 'field list'

I can just add these missing fields right?
any idea what can be wrong?

Dumb question, you did run the upgrade script right? It should handle all db changes...

Yes, make sure that you go through the UPGRADING.html file. Prior to 6.4 you'll need to manually run the altertables. The UPGRADING.html file will give you the instructions for upgrading from 6.3.1.