Handling a "Mixed Type" Matrix (String, Float64 and Int64)

General Usage
2

You could consider doing this:

julia> T = Union{String,Int}
Union{Int64, String}

julia> myMatrix = Matrix{T}(undef,12,3)
12×3 Matrix{Union{Int64, String}}:
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef
 #undef  #undef  #undef

julia> for i in 1:j
           a= i 
           b=2*i
           c = Vector_Month_Names[i]
           myMatrix[i,:] = vcat(a,b,c)
       end

julia> myMatrix
12×3 Matrix{Union{Int64, String}}:
  1   2  "Jan"
  2   4  "Feb"
  3   6  "Mar"
  4   8  "Apr"
  5  10  "May"
  6  12  "Jun"
  7  14  "Jul"
  8  16  "Aug"
  9  18  "Sep"
 10  20  "Oct"
 11  22  "Nov"
 12  24  "Dec"

Otherwise, I think DataFrames.jl is what you might want instead of a matrix.