The Sechseläuten is a traditional spring holiday in the Swiss city of Zürich. This python function calculates for a given year the holiday's date.

sechselaeuten.py

def sechselaeuten(year):
    from datetime import date, timedelta
    from dateutil.easter import easter
 
    april_mondays = [date(year,4,d) for d in range(1,31) if date(year,4,d).weekday()==0]
    holy_monday   = easter(year) - timedelta(days=6)
    easter_monday = easter(year) + timedelta(days=1)
 
    if(april_mondays[2].isocalendar()[1] in (17, 18)): # avoid school holidays...
        if(april_mondays[1] not in (holy_monday, easter_monday)): #...unless there's s.th. else
            return(april_mondays[1])
 
    if(april_mondays[2] == holy_monday): # avoid holy week
        return(april_mondays[1])
 
    if(april_mondays[2] == easter_monday): # avoid easter
        return(april_mondays[3])
 
    return(april_mondays[2]) # default date