Calculate the last day of the month using liquid only

last-month-day.liquid

{%- capture current_month %}{{ site.time | date: '%m' }}{% endcapture -%}
{%- capture future_date %}{{ site.time | date: '%Y' }}-{% if current_month == 12 %}01{% else %}{{ current_month | plus: 1 }}{% endif %}-01{% endcapture -%}
{%- capture epoc_end_month_date %}{{ future_date | date: '%s' }}{% endcapture -%}
{{ epoc_end_month_date | minus: 86400 | date: "%B" }}{{ " " }}
{%- assign day = epoc_end_month_date | minus: 86400 | date: "%-d" -%}
{%- case day -%}
  {%- when '1' or '21' or '31' -%}{{ day }}st
  {%- when '2' or '22' -%}{{ day }}nd
  {%- when '3' or '23' -%}{{ day }}rd
  {%- else %}{{ day }}th
{%- endcase -%}