import re


def remove_observed(exp_name: str | None) -> str | None:
    """Scrub (Observed) and calendar year from event names. 'Christmas Day (Observed) 2021' becomes 'Christmas Day'."""
    if exp_name is None:
        return None

    regexp = (  # Captures (Observed), YYYY, and any whitespace before, after, and in between.
        r"( *\(Observed\) *)|( *\d{4} *)"
    )
    return re.sub(regexp, "", exp_name)