You can plant container-grown trees at any time, just make sure it gets plenty of water during the (rapidly approaching) hot season.

I have several large beautiful oak and ash trees in my yard. The land in my yard is very uneven. I would like to use fill to make it even but do not want to cut down the trees. I want to keep them. I am told you cannot fill around the trees more than 1 to 3 inches per year. Some parts of the yard slope steeply about 3-4 feet. What can I do. Cutting the trees down is not an option.

Suzette,
you are hijacking someone else's thread.
Beside that, you could get a more qualified answer on either Trees or Landscape Design forums.
Anyway, terracing could be one of the possible answers/solutions.