Well, the right time depends on which part of the Is this Jolly enough black cat Christmas shirt planet you live in. For example, in the UK, Christmas decorations are usually included at the beginning of Advent season (from late November to early December), while in the US, sometimes around Thanksgiving (also takes place at the end of November).
IS THIS JOLLY ENOUGH BLACK CAT CHRISTMAS SHIRT
First, I should clarify that not all Australians eat shrimp at Christmas. The tradition of Christmas lunch / dinner is quite diverse among families. Some people look for something like you will get in Europe or America, with grilled pork, grilled ham, turkey, baked potatoes and such. Some spread cold, with things like cold meats, salads and cold seafood. Many people make barbecue with salad.
Regarding why many Australians have shrimp at Christmas, I have said for the following reasons:
They are very suitable for Christmas in hot weather because they are very flexible and can be cooked on the Is this Jolly enough black cat Christmas shirt grill or served cold.