Why not just use the month
and year
parameters?
{exp:channel:entries channel="YOURCHANNELNAME" year="2015" month="01" status="not closed" limit="1" sort="asc" orderby="date" disable="categories|pagination|category_fields|member_data"}
...
{/exp:channel:entries}
If you're on an archive page like:
http:///www.your-domain-name.com/blog/archive/2015/01
You could do this:
{exp:channel:entries channel="blog" year="{segment_3}" month="{segment_4}" status="not closed" limit="1" sort="asc" orderby="date" disable="categories|pagination|category_fields|member_data"}
...
{/exp:channel:entries}
You can read more here:
year=, month=, day=
Hope that helps!