Botswana, and along with it, the Okavango Delta, is a seasonal destination.
Broadly speaking, there are two main "seasons'' - wet and dry. The wet or green season (summer) tends to be from November to March/ April, with January and February being the wettest months.
The dry, winter season tends to be from April/ May to October. This is the time of year when game-viewing, in general, is at its best - as the wildlife congregates along water sources.
Although the Delta is a year-round destination, it is so as it attracts visitors with different interests.
For birders, the summer season is the best, the migratory species are visiting in huge numbers to breed. In addition, many of the plains game species (antelope, zebra) are dropping their young... the grass is green and lush, and (as this is nature at its most natural) there are excellent numbers of predators.