There is a Calendar Poketch app that says what day it is. It is obtainable by showing the professor in Sunyshore City a Pokemon with a serious nature.
Chat with our AI personalities
You can get it by talking to Dawn/ Lucas's sister in Pearl or Diamond. Each day they tell you a different species of Pokemon is swarming, and where to find it. The Pokemon changes each day, so Cubone will eventually be swarming, if it isn't today.
A: They day on Diamond is the same day as on your DS, as for the time of day, look at how it is in game.
It's in a town
Pokemon diamond
After you beat the Elite Four and recieve the National Pokedex, Dawn's sister tells you where a swarm of pokemon are. These are usually Pokemon from different generations of games. However, you can only talk to her once a day.