You are working on a big project. You have to work for one month for the project. At every single day of the month, you have to be very concentrated. You would have to be persistent to finish the work.
However, if in the beginning of every day, you decide to be concentrated for only one day. The task would be seem to be lighter. The psychological pressure would be much lower.
Then, in the beginning of another day, you decide to be concentrated for only one day again.
Using this mechanism, you just have to be persistent for one day at every time, much easier than to be persistent for the whole month. However, you can still get the benefit of being persistent for the whole month.