Where can I make a 12 month multi picture calender online professionally?

[Nahum]

Not sure of any specific sites, but you can find "collage calendar templates" in quite a few places. It would also be a relatively simple project to make in desktop publishing programs like Microsoft Publisher, Open/LibreOffice Draw, or Scribus.

Making it "professionally" would be a matter of using high resolution photos (minimum size of roughly 1–2 megapixels), printing from high quality large format printers, and using durable poster stock. A print shop would certainly be a good choice for output, and they could potentially laminate the poster or use some printing method that would allow the calendar to survive the year.