Translation:The first of April is an interesting day.
At least in the US, I doubt anyone would use "First April is an interesting day." In speech or in writing, it would be "THE first OF April is an interesting day," as in the translation above. Maybe the date -- by itself -- would sometimes be written as "1st April," but here the usage is in a sentence, where I don't think it works. But this is only my opinion.
So you don't usually spell out the number as it is in this sentence. The most common is to write 1. dubna 2017 (read "prvního dubna"), sometimes 1. duben 2017 ("první duben"). The dot that follows the number indicates the number is ordinal (first, second,...).
You can also just write the number of the month, that's very common, too: 1. 4. 2017, 01. 04. 2017, 01/04/17... (read all of these as "prvního čtvrtý dva tisíce sedmnáct"). There are multiple options but always in the order day, month, (year).