Can
anyone tell me when the best time would be to carry this out? I am not

worried whether they will live or die in their new position but I have
no more room for another container and there is space in the garden.

I have found that freesias grown in a pot will happily transplant to
the garden once their flowers have gone over. Cut off the flower stems
so they don't waste energy trying to grow seeds (you can let them do
that next year) and plant them where you are happy for them to spread.
I give them a dilute liquid feed as long as the leaves are green, then
forget about them until they start to show leaves through the soil, in
my case sometime in November.

Jo
On a south facing slope on the Mediterranean coast of Spain