How to pass a parameter into a function
I suggest an SQL function:

CREATE OR REPLACE FUNCTION foo(_date date)
  RETURNS TABLE (
   name       text  -- types have to match your actual types!
 , keyword_id int
 , project_id int
 , the_date   date
 , today      int
 , yesterday  int
 , week       int
 , month      int) AS
$func$

SELECT k.name, f.keyword_id, f.project_id, _date -- AS the_date -- col alias irrelevant
     , f.t AS today, f.y As yesterday, f.w AS week, f.m AS month
FROM   crosstab(

    -- crosstab function from previous question here
    -- http://dba.stackexchange.com/a/71266/3684

   ) f (rn int, keyword_id int, project_id int
       , t int, y int, w int, m int)
JOIN   keyword k USING (keyword_id);

$func$  LANGUAGE sql;


Call:

SELECT * FROM foo('2014-07-07');


Replace all occurrences of now()::date with your input parameter named _date.
 In Postgres 9.1 or older use the positional parameter $1 in SQL functions (which can be used in any version).

More code examples:


Return rows from a PL/pgSQL function
Pass In “WHERE” parameters to PostgreSQL View?


Tricky detail

The crosstab() function takes query strings as parameters. Function parameters are not visible inside crosstab(). So you need to pass in the date values as string literals!
I suggest the function format() for convenience. For example, the second parameter becomes:

,format('VALUES(%L::date), (%L), (%L), (%L)'
        , $1, $1 - 1, $1 - 7, ($1 - interval '1 month')::date
       )


instead of:

,$$
 VALUES
       (now()::date)
     , (now()::date - 1)
     , (now()::date - 7)
     , ((now() - interval '1 month')::date)
 $$


Complete code

SQL function

CREATE OR REPLACE FUNCTION foo_sql(_date date)
  RETURNS TABLE (
   name       text
 , keyword_id int
 , project_id int
 , the_date   date
 , today      int
 , yesterday  int
 , week       int
 , month      int) AS
$func$
BEGIN

SELECT k.name, f.keyword_id, f.project_id, _date
     , f.t, f.y, f.w, f.m
FROM   crosstab (
   $$
   SELECT rn
        , pk.keyword_id
        , pk.project_id
        , d.created_at
        , COALESCE(pr.pos, 0)
   FROM  (SELECT *, row_number() OVER () AS rn FROM project_keyword) pk
   CROSS  JOIN (
       $$
    || format('VALUES(%L::date), (%L), (%L), (%L)'
              , $1, $1 - 1, $1 - 7, ($1 - interval '1 month')::date)
   || $$
     ) d(created_at)
   LEFT JOIN (
      SELECT keyword_id
           , project_id
           , created_at::date AS created_at
           , min(position) AS pos
      FROM   project_report
      GROUP  BY keyword_id, project_id, created_at::date
      ) pr USING (keyword_id, project_id, created_at)
   ORDER  BY pk.rn, d.created_at
   $$

  ,format('VALUES(%L::date), (%L), (%L), (%L)'
              , $1, $1 - 1, $1 - 7, ($1 - interval '1 month')::date)
   ) f (rn int, keyword_id int, project_id int
       , t int, y int, w int, m int)
JOIN   keyword k USING (keyword_id);

END
$func$  LANGUAGE sql;


PL/pgSQL function

Shorter, reusing the VALUES expression. Probably a bit faster, too.

CREATE OR REPLACE FUNCTION foo_plpgsql(_date date)
  RETURNS TABLE (name text, keyword_id int, project_id int, the_date date
               , today int, yesterday int, week int, month int) AS
$func$
DECLARE
   _dates text := format('VALUES(%L::date), (%L), (%L), (%L)'
                        , $1, $1 - 1, $1 - 7, ($1 - interval '1 month')::date);
BEGIN

SELECT k.name, f.keyword_id, f.project_id, _date, f.t, f.y, f.w, f.m
FROM   crosstab (
  'SELECT rn
        , pk.keyword_id
        , pk.project_id
        , d.created_at
        , COALESCE(pr.pos, 0)
   FROM  (SELECT *, row_number() OVER () AS rn FROM project_keyword) pk
   CROSS  JOIN (
       ' || _dates || '
     ) d(created_at)
   LEFT JOIN (
      SELECT keyword_id
           , project_id
           , created_at::date AS created_at
           , min(position) AS pos
      FROM   project_report
      GROUP  BY keyword_id, project_id, created_at::date
      ) pr USING (keyword_id, project_id, created_at)
   ORDER  BY pk.rn, d.created_at'

  ,_dates
   ) f (rn int, keyword_id int, project_id int, t int, y int, w int, m int)
JOIN   keyword k USING (keyword_id);

END
$func$  LANGUAGE plpgsql;


CTE

For completeness, the "same" without persisting a function, with CTEs:

WITH d(day)   AS (SELECT '2014-07-07'::date)  -- provide your date here
,    v(dates) AS (
   SELECT format('VALUES(%L::date), (%L), (%L), (%L)'
                 , day, day - 1, day - 7
                 ,(day - interval '1 month')::date)
   FROM d
   )
SELECT k.name, f.keyword_id, f.project_id, d.day AS the_date
     , f.t AS today, f.y As yesterday, f.w AS week, f.m AS month
FROM   crosstab (
  'SELECT rn
        , pk.keyword_id
        , pk.project_id
        , d.created_at
        , COALESCE(pr.pos, 0) AS pos
   FROM  (SELECT *, row_number() OVER () AS rn FROM project_keyword) pk
   CROSS  JOIN (
       ' || (SELECT dates FROM v) || '
     ) d(created_at)
   LEFT JOIN (
      SELECT keyword_id
           , project_id
           , created_at::date AS created_at
           , min(position) AS pos
      FROM   project_report
      GROUP  BY keyword_id, project_id, created_at::date
      ) pr USING (keyword_id, project_id, created_at)
   ORDER  BY pk.rn, d.created_at'

  ,(SELECT dates FROM v)
   ) f (rn int, keyword_id int, project_id int
       , t int, y int, w int, m int)
JOIN   keyword k USING (keyword_id);