〇〇歳になる前日は、「The day before you turn xx」。
例えば、The day before you turn 32.
質問者様のお求めの英語は以下の通りです:
1: If you receive the working holiday visa approval by the day before you turn 31, it's fine.
2: There are only 1000 slots available for UK working holiday visa.
3: There's no limit to how many working holiday visas in Australia.