drinkを具体的な名前に置き換えてください。
Come in costume and your first glass of Heineken is free（コスチュームで来た方には、ハイネケン最初の一杯が無料です）
～一杯＝a glass of～, a cup of～
「最初の」とは言っていませんが、2つ目のような文章もポスターで見かけます。
Wear your pizza costume to our store and receive 1 free slice!（ピザのコスチュームで来店した方には1枚無料！）
小さい数字はスペルするよう習っているかもしれませんが、ポスターに書かれる場合、アラビア数字を使っても大丈夫です（そのほうがわかりやすいので）。freeを大文字でFREEとして強調することもできます。
ご参考になれば幸いです。
Wear a Halloween costume and your first drink is free!
Surely, customers will be well pleased to know that they qualify for a free drink if they wear a Halloween costume! The format: OFFER + CRITERION or CRITERION + OFFER is typical of advertising placards and offers.
Eg: "Buy one, get one free!"
'
確かに、ハロウィーンのコスチュームを着れば、最初のドリンクが無料と聞けば、お客さんは十分に喜ぶでしょう！フォーマット：OFFER + CRITERION or CRITERION + OFFERは、広告プラカードやオファーの典型的なフォーマットです。
例：
Buy one, get one free!
（一つ買えば、一つ無料！）
When talking about a drink being free it is always good to clarify which one so you would wrire 'first drink free' meaning only the first drink will be free
If you put clothing on you are said to 'wear' it
a costume is usually designed for an event that is happening so halloween would be a ghost witch etc
Come enjoy a free drink if you are dressed in a costume!
We can use the above sentences for the Halloween poster.
Halloween is great fun, a day where everyone can be what they feel like and hide behind a mask.
Halloween is an age-old tradition celebrated in many parts of the world.
Many public places make an event out of this day and frequently give away prizes and other free things.
Participants: people that take part/join in
It's a Spooky Halloween! One Free Drink For All Monsters!
Halloween Night! Costume Party! Free First Drink.
Halloween is all about dress up, so people know they get to dress up in costumes. This allows everyone to dress up in the costume they feel like and they get to have a free drink as well.
Many people associate with Halloween as being spooky. When you say "spooky", people know that they get to dress up in costumes and that they are going to have a party.
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