It's not an idiom. Suppose there is something that you want. Eg to start a new job, buy a new computer, start going to a gym. All these things can be difficult and costly so you need extra motivation. You say to yourself "I will start a new job in the new year." The new year isn't the real reason, it is the explanation that you are using to do the difficult or costly thing. "New year" means "January the first" (as a date on which many people will start new things)
If you have been thinking about changing your job, let the new year be your excuse!
I had been using cheap shampoo, but I let the new year be my excuse for buying some expensive bath oils.
You know that you will need to buy a new car eventually. Let the new year be your excuse for trading in your old car and getting a brand new one.
New year is a good time to think about your future. Let the new year be an excuse for making a change for the better by getting fit and eating well.