user: I'm planning an international conference and need to consider public holidays when scheduling. Could you retrieve the list of holidays for the United States in 2018, Germany in 2020, Spain in 2019, and the United Kingdom in 2021? It's crucial these dates are accurate to avoid any clashes with national holidays.

Available functions:
name: retrieve_holiday_by_year
description: Finds the holidays of a year.
parameters: type: dict
properties: year: type: string
description: The year of the holidays.
country: type: string
description: The country of the holidays. Possible options: US, AT, DE, ES, FR, GB, IT, NL, PL, RO, SK, UA.
required: year
country