There are certain times of the year that is best to visit a city. However, it depends upon the reason someone is wanting to come and visit a certain city. If you are wanting to visit a city when everything is going on, then you should visit during their peak season.
If you are wanting to visit the city when it is quiet and not crowded, then visit during their non-peak season. If you are going sightseeing, then there is not a certain time of the year because that is available all of the time. If you want to visit Bangkok for the culture, it might be best to go when there is a festival. Therefore, mid-April is the best time to visit Bangkok for the Songkran festival.