There are so many variables. I’ll try to narrow down the options.
If it is a popular, commercial, wedding venue it is not uncommon to book 12-18 months in advance, unless there is a cancellation. Some may even require longer.
At some popular sites, depending on the size of your party, many larger venues can hold more than one function at a time.
PARKS & HALLS
However, if it is a park, beach, community hall or club, you may be able to book in only weeks or perhaps a few months in advance.
For a back yard weddings, that is entirely up to you.
Spring tends to be the busiest time, followed by Autumn and Summer and finally Winter and so if you want a Winter wedding, you will have more options and flexibility and maybe even a lower price.
You will find a list of options for Venues on my Venues tab.