Doesn't it sound weird to say "this garden" instead of "a garden"?
When speaking, many native speakers tend to replace "a" with "this." "This" functions in essentially the same way as "a" and can be used interchangeably. "A" is used in writing ("this" is considered too informal for writing), and "this" might be used when speaking, to replace "a." Generally, using "this" in this way is quite casual and cannot be considered standard English. Additionally, "this" is used to refer to specific things. The speaker is mentioning a specific party, so "this" was used here. Ex: There was this [a] huge golden retriever at my friend's house yesterday. Ex: I'm so sorry I'm late! I was stuck behind this [a] guy going only 50 kilometres an hour.