SQL Left Join losing rows after filtering

First of all i don't think it is a good idea to have number as an alias. Use a character or a word instead.

I would put the criteria in the join.

SELECT T1.Label, COUNT(T2.values)
FROM Table1 T1
    LEFT JOIN Table2 T2 ON T1.fk = T1.pk
    AND T2.Date BETWEEN '20100101' AND '20101231'    
GROUP BY T1.Label

Other comments:

1. I would use ANSI(yyyyMMdd) format for the dates, so there is no missunderstanding.
2. I would be aware of the BETWEEN as it is not clear what you want to do if the date is '20101231 01:00' Would you like to include that date or not? >= and <= or >= and < is clearer.

You are filtering on the second table in the where. The values could be NULL and NULL fails the comparisons.

Move the where condition to the on clause:

SELECT 1.Label, COUNT(2.values)
FROM Table1 1 LEFT JOIN
     Table2 2
     ON 1.fk = 1.pk AND
        2.Date BETWEEN 1/1/2010 AND 12/31/2010    
GROUP BY 1.Label

Note:

The date formats retain the dates from the question. However, I don't advocate using BETWEEN for dates and the conditions should use standard date formats:

SELECT 1.Label, COUNT(2.values)
FROM Table1 1 LEFT JOIN
     Table2 2
     ON 1.fk = 1.pk AND
        2.Date >= '2010-01-01' AND
        2.Date < '2011-01-01'
GROUP BY 1.Label;

Some databases support the SQL Standard keyword DATE to identify date constants.

simply move the condition in WHERE clause to the ON clause.

LEFT JOIN Table2 2 ON 1.fk = 1.pk AND 2.Date BETWEEN '1/1/2010' AND '12/31/2010'