Zeller's congruence algorithm in Ruby

zeller.rb

# day is Numeric
# month is String
# year us Numeric

def zeller(day,month,year)
    
    days = ['Saturday','Sunday','Monday','Tuesday','Wednesday','Thursday','Friday']
    
    case month
    when 'January'
        monthind = 13
        year -=1
    when 'February'
        monthind = 14
        year -=1
    when 'March'
        monthind = 3
    when 'April'
        monthind = 4
    when 'May'
        monthind = 5
    when 'June'
        monthind = 6
    when 'July'
        monthind = 7
    when 'August'
        monthind = 8
    when 'September'
        monthind = 9
    when 'October'
        monthind = 10
    when 'November'
        monthind = 11
    when 'December'
        monthind = 12
    else
        puts 'Invalid month'
    end    
    
    monthind += 1
    monthPart = (monthind*26)/10
    
    yearPart = year + year/4 
    yearPart += (6*year/100) 
    yearPart += year/400
    
    h = (day + monthPart + yearPart) % 7
    days[h]
end

zeller(4,'January',2014) # "Saturday"