What causes different locations to get their peak season for thunderstorms at different times of the year like some locations get the most thunderstorms in a July some in June and some in August and some get the most thunderstorms in September. What causes the peak season for thunderstorms to be different in different locations?
What you are really asking is why the weather varies from place to place, sometimes in places which are only a few miles apart (in the hills of the Dhofar region of S.Oman, you have a transition from monsoon scrub and grassland to arid desert in the space of only 2 or 3 hundred metres!). Weather is such a complex thing it is impossible to enumerate all the factors which cause local variation, except to say that the presence of hills and mountains often has something to do with it.
Matter is made up of sub-atomic particles containing positive and negative charges. They are usually evenly distributed, every positively charged particle being balanced by a negatively charged particle, which means these charges cancel each other out. Nearly everything we see around us is therefore electrically neutral. This disguises the enormous power of the electromagnetic force, which is trillions of times more powerful than the gravitational force, so even a very slight imbalance between positive and negative charges, such as occurs in thunderstorms, can produce dramatic results.
These electrical imbalances can be produced by air turbulence such as takes place in rising plumes of hot air called thermals, of which the strongest happen in summer. Particles of dust or moisture in the rising air currents rub against each other and cause separation of charge. The condensing raincloud becomes negatively charged, while the ground below becomes positively charged. Positive and negative charges hate to be separated, as they have a very strong attraction for each other.
The positive charges on the ground are pulled along by the negatively charged cloud, and tend to build up on church steeples and similar pointed objects that reach into the sky. When the two masses of separated charge become sufficiently strong, and especially if they pass over a tall, pointed object, they cannot resist the urge to recombine and an electrical discharge takes place. Why one place should have more of these events in August and another in September is a complex question which cannot be answered without knowing all the many variables which influence the weather in both of these places.