We are hitting the main artery of the season, with races nearly every weekend, from sprints to Ironman. Athletes are training and preparing for the big races of their season, which likely happen in the summer or early fall.
Most athletes doing Ironman make it their clear "A-priority" race of the season, as the distance and sheer demands of the race force them to do so if they hope to have an enjoyable experience. This includes half-Ironmans for many athletes as well.
Whether your big race of the season is an Ironman, half-Ironman, Olympic or Sprint, there are training principles which apply and need to be adhered to, in order to meet your potential for the race.
Hopefully you are training with a general plan of periodization, where you change the training stress of the body as the season progresses, and you get closer to the big race. If you're doing the same workouts at week 1 and week 18 in your preparation, you're likely hitting a plateau and not seeing much, if any, more improvement. The body will plateau and become stagnant if you don't change the training stress.
How do you change training stress? There are only three aspects you can change in your training; frequency, duration and intensity. Notice, I did not say volume, as volume is simply frequency times duration, a product of the two.
Many athletes don't change any of those, running thru the same training routine week after week. Periodization involves changing the stress in many different ways, which includes all three aspects, as the calendar moves toward the race.
So when and how should we change the training stress? This depends entirely on the athlete, their goals, their training history, time to prepare for the big race, and even the length of the race. For example, simply adding more volume, with light, aerobic intensity is probably fine for a beginner attempting their first Olympic, Half or even full Ironman, if the time frame allows this.
The most important concept of periodization is as the time for the big race gets closer, athletes need to make sure their training more closely mimics the demands of the race. The demands of the race are in accordance with the goals of the athlete and need to be well understood, so the athlete can better simulate these in training.
So when should this happen? This is the key question, because there is an important training window which will greatly control the result of the race. Depending on the athlete, the general window of time which has the biggest affect on the race is from 12 to 3 weeks before the race. This will be the time the body adapts and prepares best for the specific demands of the race. There will likely not be a plateau, and you'll still have enough time for a taper if attempting a longer event.
Pay attention the specific demands of your race, based on your goals, and be sure to make your training more closely mimic them in that 12 to 3 weeks out window.
Best of luck!