Pomeranians generally don't get their period (aka "have their season") until they're around two years old. Females are sexually mature at about two to three years of age, though often many owners will commence spaying or neutering them before this time frame because the breeds aren't used for breeding purposes.
A few things can contribute to early-onset periods (just like in humans), including living with other large female dogs, getting pregnant, and certain health problems like Hypothyroidism. It's also possible for these pups to go through what is called reverse seasonality-meaning that then they might not have a period or be very irregular throughout the year if they live somewhere really cold all the time...