在一年的这个时候是译成at this time of a year还是at this time of year 还是at this time of the year?

在一年的这个时候是译成at this time of a year还是at this time of year 还是at this time of the year?
主要就是那个冠词,是a,还是an,还是没有
thank you all the friends who gave me the answer