According to what I could find online, it seems there are only a few mughal gardens that are only open once a year. This would most likely be to keep the gardens original integrity and design from being completely ruined by tourists. (Many historical sites of significance are now no longer open to tourists period because of the bad behavior of a few of them; Machu Pichu is a good example of this.)
However, the mughal gardens are a collection of gardens in India, Pakistan, Bangladesh, and Afghanistan. While some of these destinations may only be open once a year in any country, some of them are open year round. Just be careful not to destroy that experience for others.