Cook, Serve, Delicious! 2!! (UP)

If like me, you don't really play this mode much other than to get the associated achievements with the CSD mode and you just play C4H mode, you can grind out the remaining days you need towards your 20 days with little interaction.

You can just use Zen mode
Prep Stations: 4
Holding Stations: doesn't matter but I had it set to max (8).
Buzz: doesn't matter really because you won't be doing much.
Infinite Patience: ON

Have a menu which is completely empty except for an easy auto serve item like Beef Brisket slices, you don't need to have Sides or Drinks at all, just leave them empty.

Start a new day then just start to prepare Beef Brisket slices so that the clock starts from 6:00am. Now just leave the game idle for about 4 mins, nothing will happen because of the infinite patience and 4 Beef Brisket orders will just sit in the Prep Stations.
When the clock gets to about 7:00-8:00pm, make sure you prepare some Beef Brisket slices for when 10:00pm comes and the "finish" occurs. You can't prepare them earlier in the day because otherwise they will spoil.

Just serve the 4 Beef Brisket orders after 10:00pm then the day will finish and this will count 1 day towards the 20 and will take about 5 mins of your time.
You can tell how far you are on this achievement because when you start a new day there will be a Bonus Objectives screen before you start which will have "Complete a Day at CSD! in Any Mode (X/X+1)" where X will be how many days you have completed, the objective will always be to complete an additional day. You just need to get to 20 though for the achievement.