Over time, I've realised that certain words you use when referring to time, such as ’今日’, ’明日’ and ’毎朝’ have no need for you to attach the に particle. However, when saying '六時’, you must attach a に.
For example, ’今日、学校に行きません。’ and ’毎日午前六時におきます。’
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You should use に when the 'time' word means a particular time, for example, 六時 which is 'six o'clock.' You don't need to, however, if the word implys a period, such as 今日, and the action word can mean a kind of status.
Generally, one uses に when talking about frequency of an action or a specific point in time (eg. 8 am). に isn't used for general concepts like 'today' or 'yesterday'