The language Raku (formerly known as Perl6) comes with a date class built in. And there is no need to be scared. You can immedeatly understand what this does:

constant \SUNDAY = 7;

.say for
    ( 2019 .. 2100 )
    .map(  { DateTime.new( :$^year, month => 12, day => 25 ) })
    .grep( { .day-of-week == SUNDAY })
    .map(  { .year })
;
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I am just posting this because you mentioned 6, it is not related to the problem (obviously).