Express when festivity or event happens
Of course you can put the event before or after the date.

When you write the name of an event first, you can add the information of when this event occurs.
Example: [圣诞节] [是在] [十二月二十五日] = [Christmas] [is on] [25th December]

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When you write a date first, you can add the information of what this date represents
Example: [十二月二十五日] [是] [圣诞节] = [25th December] [is] [Christmas]

More example:
我的生日是在日十二月二十五日 - My birthday is on December 25th.
十二月二十五日是我的生日 - December 25th is my birthday

When you are describing &quot;when the event occurs&quot;, you can omit &quot;在&quot; and write &quot;圣诞节是十二月二十五日&quot; (Christmas is 25th December) or &quot;我的生日是十二月二十五日&quot; (My birthday is 25th December)

When you are describing &quot;what the date represents&quot;, you cannot add &quot;在&quot;  and write  &quot;十二月二十五日是在圣诞节&quot; (25th December is on Christmas) or &quot;十二月二十五日是在我的生日&quot; (25th December is on my birthday )

You can add more information about the date. &quot;十二月二十五日&quot; just means &quot;25th December&quot; ; &quot;每年的十二月二十五日&quot; means &quot;25th December of every year&quot;