What is the function of 先 in 我諗聽日先買飛嘅話一定買唔到 (cantonese)

I don’t think (you/we)’ll be able to buy a ticket tomorrow.
我諗聽日買飛嘅話一定買唔到。

I don’t think (you/we)’ll be able to buy a ticket if (you/we) wait until tomorrow.
我諗聽日先買飛嘅話一定買唔到。

Yes, it can be discarded. You can say 我諗聽日買飛嘅話一定買唔到 and the meaning is the same. I'm not sure if it's "until", but I feel it's similar to "才“

先 here is short for 先至

• 先(first)

• 至(then)

先至 (first then)= till... then

我諗等到聽日(先)買飛嘅話一定買唔到

I think if we wait (till) tomorrow (then) to buy ticket we will certainly fail to buy any

The word "等到" (wait) is implied

Example of difference between "先至/ 先" (till...then) and "先" (first/ for now)

做好功課(先至/先)准睇電視 - (Till) finish all the homework (then) allowed to watch TV.

你地坐吓等陣(先) - you guys sit for a while and wait (first/ for now)

• When "...先" is followed by a verb phrase, it is a short form of 先至, meaning "till ... then"

• When 先 is at the end of a words clause, it means "first" or "for now"