Hi ya,
You can book in advance for example we get married on the 17th May and we need to book the registrar to attend the wedding venue on that date, when we went last week they told us the bookings start in January and so we are going back on the 15th January to pay the 100 dinar fee and book the date, like I say not sure if its the same all round as I know that each registrar/muncipality in each area has their own rules! but this was the advice based for Tunis.
Definately get your H2B to investigate, though if you are planning to come over before then maybe do it yourself, I live here and its still like drawing blood out of a stone to get H2B to organise some things, its all tomorrow, tomorrow, though hes finally hit the panic stage as we come closer to it
Take care hun x