Fading LED Circuit

GOAL- My goal is to light up a small model I have using an LED circuit that will automatically fade in and out.

DESCRIPTION - I will be using 19 blue LEDs (3-3.2V fwd voltage, 20mA fwd current) and would like them to fade in and out on their own very slowly. I am thinking that this will be powered using a phone charger or other household transformer since batteries don't seem to be a feasible option for this (too many batteries clustered together and/or too short a life span). I have a transformer on hand that runs 5 volts at up to 850mA, so if that works then perfect, if not then I could try to find another transformer. For the sake of simplicity I could add one more LED to make it an even 20, though I would have to drill another hole in my model.

TASK - I need the circuit to make these LEDs do their fading. Simple as that. Fading circuits are easy to find on the internet, but I have been having trouble finding anything to meet my project criteria. Any help with this would be immensely appreciated!