[Tagging] Is there some established tag for free days at museums (and other object)?

[Mateusz Konieczny]

Is there some established way to tag things such as

- museums fees are waived on Tuesdays
- museums fees are waived on Sundays, only for standard expositions
  (temporary expositions are still paid)
- museums fees are waived on winter Sundays, only for standard
  expositions (temporary expositions are still paid)

Note that in this case I at least plan on tagging such objects in my
city and then making map from OSM data and sharing it, so it is not
100% abstract proposal for mapping something without use. 

I strongly prefer to avoid putting some complicated conditions into fee
tag as I think that fee tag should contain binary yes/no value (is it
typically paid) to make it usable without requiring full blown opening
hours parsing.

I thought about tagging it with two tags

- free_entrance tag, with syntax the same as opening hours
- free_entrance_coverage, with "full" for unlimited free entrance and
  "unchanging_exhibition" for free entrance not covering changing
  exhibition (probably there is a better name for that). I expect that
  free_entrance_coverage may need also other values in other regions.

For somebody wishing to tag more complicated situation, one may use
additionally.

free_entrance:2, free_entrance_coverage:2
free_entrance:3, free_entrance_coverage:3
free_entrance:4, free_entrance_coverage:4

tag pairs to tag things like "left wing has free entrance on Tuesdays,
right wing has free entrance on Saturdays".