The calendar promotions are organized with daily mobile casino deals for specific day of the week. Most of them are organized for the weekend when most players gamble with mobile. It's a popular practice to see such deals for the beginning of each week or month.
Many calendar casino offers use comp points, VIP rewards, bonus levels and some specials. It's good to participate in such promotion at the very beginning because you can earn more points, bonuses and get higher position on the final leaderboard.