(n.) a day or one day; (slang) used as part of a prepositional phrase (一日都係) to blame someone (or something or 'a reason') for some negative things that have happened, all because of; owing to; thanks to (e.g. 一日都係你 means "It's all because of you" or "Thanks to you".)
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See how to write "一日" with stroke-by-stroke animation