Fetching details from the dictionary...

assumption (of office); taking up (a post); inauguration; installation
新社長が就任して方針が変わった。
The new company president took office and policies changed.
就任式で挨拶をすることになった。
I will give a speech at the inauguration ceremony.
大臣に就任したばかりの頃のことだ。
He had just taken office as minister.