In the UK, zoos like to have a big new attraction built each winter to open in spring and attract visitors are in the summer. I am sure the same applies around the world. But there are other improvements that matter too.
A well managed zoo should aim to improve every month and indeed every week. Not every improvement will be obvious, some will be so small that they are barely noticeable individually - but they should build into a pattern of progress and development which motivates the different teams of staff and does create an atmosphere which visitors will sense.
A revised menu in the restaurant, a resurfaced path, a replanted flower bed, new branches to provide better opportunities for climbing in a monkey exhibit, better lighting for an aquarium, a more informative label on an exhibit, new enrichment to make a cat more active, better staff training and so on; they all add up.
There are some improvements that are essential, but everyone hopes that they will never be necessary: good zoos regularly revise and improve their plans and drills for all the disasters that might happen - escapes, disease outbreaks, fires and other emergencies.
Of course there will be setbacks - favourite animals get sick and die, experienced staff retire or move on, bad weather reduces visitor numbers - these are unavoidable: good zoos are resilient enough to bounce back quickly. Disasters are different: good planning can mitigate some of the problems caused by floods, typhoons and earthquakes, but major damage can be inevitable.
Alan