>If I have this:
>
>

>SELECT SUM(DATEDIFF(d,Holiday.Start,Holiday.End))
>

>
>...I can obtain 365.
>
>Now, if I change the type to s, to obtain this same amount in seconds:
>
>

>SELECT SUM(DATEDIFF(s,Holiday.Start,Holiday.End))
>

>
>...I obtain 23533200.
>
>However, If I do 23533200/84600, I do not obtain 365 days but only 272.38.
>
>What am I missing?

I am not sure what are you missing, I am sure missing something as my test works fine:

declare @Holiday table ([Name] varchar(32), [Start] datetime, [End] datetime) 

insert into @Holiday ([Name], [Start], [End]) values ('Christmas', '2014-12-24', '2015-12-24')
insert into @Holiday ([Name], [Start], [End]) values ('New Year', '2014-01-01', '2015-01-01')

SELECT [Holiday].[Name], SUM(DATEDIFF(d, [Holiday].[Start], [Holiday].[End])) from @Holiday Holiday group by [Holiday].[Name]

-- Christmas	365
-- New Year	365

SELECT [Holiday].[Name], SUM(DATEDIFF(s, [Holiday].[Start], [Holiday].[End])) from @Holiday Holiday group by [Holiday].[Name]

-- Christmas	31536000
-- New Years	31536000