Window Message

I'm staring at the block of apartments opposite my own, or more specifically at a group of four windows.

Each window belongs to a different apartment. The top left one has the letters H, A, P, P, Y arranged out, one letter to a single A4 sheet, across its length. The window next to it bears a message which is harder to read - as it is handwritten using a thin marker on more A4 paper - but a little squinting proves that it spells "BDAY". The bottom left window, just like the one at the top left, also has a printed message, and reads "S'PORE". The last window, at the bottom right, has no writing. It simply has a large and familiar red-and-white flag stretched out across it.

Happy birthday, indeed.