Full answer
The deposit is what secures your date: once it is paid, that day is no longer offered to other brides. That is why, if the wedding is cancelled, the deposit is non-refundable, except in very specific cases.
When the deposit is refunded after a cancellation
It is only refunded when the wedding is cancelled for a serious reason: a death, a serious accident or force majeure. In any other case it is not refunded, because the date was kept for you and could not be offered to another wedding.
What happens to the deposit if the date changes
If the wedding moves to another date, the deposit stays the same and carries over. What cannot be guaranteed is availability on the new day, even if it is only a few days away. If the new date is free, the deposit moves to it; if it is not, the deposit is not refunded.
So if you are thinking of changing the date, tell me as early as you can, so availability can be checked before you decide.
