The best thing to do, is whilst the game is running, every second, update a variable to the current date, and make sure to save this so this can be loaded in future.
Then, say 3 weeks pass and you open your game, you will have the old date from 3 weeks ago, and you can compare it to today's date, and with these two dates, you can figure out how many Sundays have occurred, and add this to a "number of Sundays" variable.