If you click on the enveloppe and scroll down to hayday events week # it'll show you what's happening that week and you can see when those 2x coins are coming up!There’s one for town visitors, farm visitors and i think truck/boat orders!
When i see a town visitor 2x coins, I keep them waiting for a few days so they can accumulate for the event day and then collect them all at the same time with a 50% coin booster.
For farm visitor 2x coins, plant A LOT of your most expensive tree/bush a few days before and harvest them as much as you can, the farm visitors often ask you for things you have a lot of and i also activate a 50% coin booster here too :)
Try to always keep your machines running making goods so you can sell them in your shop, figure out which ones are worth more and make lots of them!
And the most basic one, grow and harvest wheat and sell it full price. believe it or not people do buy it
Hope this helps!