in full swing  

(活動) 漸入佳境;正順利進行中

explanation : 
Something that is completely underway or is now at its peak. It can be used to refer to a party, a concert, a holiday season or any other event that takes place over time. Usually refers to something that starts out slow and then becomes more intense. 

e.g. When the football season started, not too many people came to the games.  But now the season's in full swing and the stadium is full.