Whilst there are some types of plants you have to plant during the planting season (winter), such as bare root and rootballed specimens, container grown trees can be planted at any time of year.
The main drawbacks of container grown specimens are a) a (slight) premium cost to rootballed/bareroot plants, b) especially with larger trees there is less choice, c) some containers encourage girdlng roots i.e. black HDPE containers (Air Pot containers are the ones that encourage the best root systems).
It is always more favourable to plant early in the planting season, this provide plants with a longer period to establish prior to the following summer. However the advantage is incremental and planting at less optimal times, (even in Spring / Summer) can be compensated by marginal increases in aftercare.
You can therefore plant at any time of year and are not restricted to the November-March planting season. Please remember if trying to plant just before the end of the planting season, it is not a date in the calender, it is when nature decides that a combination of longer daylight and temperature tells it that it is time for spring!