The example cited in the page of instruction regarding ''Expressing intervals of dates and times in French = from ... to' is 'The festival lasts from the 24th of July to the 5th of August'/'Le festival dure du 24 juillet au 5 août'. 

I am therefore puzzled by the answer to the fill-in the-blank question 'Le festival dure ________ mai/The festival lasts from the third to the tenth of May'. My answer was 'du troisième au dixième', while the answer provided as a correction was 'du trois au dix'

Perhaps I'm being stupid, but my answer seems to follow the example better than the correction. Where is my error?