When you choose for ‘in the day’ (= in the frame), a ‘stripe’ of light can simmer through the window and the frame into the room. Especially when you want to have little light coming into the room, it’s reccomended to assemble the roller blind ‘on the day’ (= on the frame).

Don’t forget to consider any obstacles, like:  window sills, a latch or extension arms. Measuring = knowing! Checking beforehand saves a lot of annoyance afterwards. Preparing yourself is half of the work.