Look into composite formatting:
Console.WriteLine("{0}.{1}.{2}", mon, da, yer);
You could also write (although it's not really recommended):
Console.WriteLine(mon + "." + da + "." + yer);
And, with the release of C# 6.0, you have string interpolation expressions:
Console.WriteLine($"{mon}.{da}.{yer}"); // note the $ prefix.