Easter has no fixed date.

复活节无固定日期。

youdao

The Spring festival is the first day of the first lunar month each year. However, the Spring festival does not have a fixed date on the Gregorian calendar.

春节是在农历每年的一月一日，但在公历上，春节日期却不是固定的，大约在公历的一月下旬到二月上旬这个阶段。

youdao

Has the date of the next meeting been fixed?

下次会议的日期确定了没有？

《牛津词典》

The Super Bowl is scheduled on a fixed date which is published years in advance.

FORBES: How Super Is the Super Bowl?

This is expected in the first two weeks in September but doesn't appear to be a fixed date in the calendar.

BBC: MPs and "armchair reshufflers" ponder cabinet changes

Interestingly there are those who oppose the weekend idea and advocate its return to fixed date, including the Veterans of Foreign War (VFW).

FORBES: Let's Not Forget What Memorial Day Weekend is Really All About - The High Cost of Freedom