Should I start a 401K account now before the end of the year or wait until January 1st?

I started with my company 6 months ago. They were in the middle of a merger and I wanted to wait until the merger finished and they decided which 401K program they were going to use. Now it has been decided, but I'm not sure whether to wait until Jan 1 to start contributing or just start now. Which option is best for tax purposes?