Ground school can be joined at any time. Flight training starts as soon as your instructor is assigned. The pacing and schedule of the lessons are up to you. You can fly once a month, once a week, or 4-5 times a day if it’s productive for you. The more often you fly the sooner the course will be done and the less money it’ll cost you to learn to fly.
Although you can begin flying at any time during the year, it is usually best to begin during the months of March to May. This way, you can do all of the single engine training in good weather, so not many flights will be cancelled due to low ceilings. By the time the rainy season comes, you will be into your Instrument rating, and you will build some valuable actual Instrument Meteorological Conditions flight training time!