The wavelength day calibration consists in using the MIDI heated blackscreen and taking 3 exposures through different narrow-band filters [NeII], [SIV] and [ArIII] (having central wavelengths accurately measured in the laboratory by the manufacturer). An extra exposure is taken through a polycarbonic foil that has several absorption lines in its N-band spectrum. Exposures without filters and with closed shutter are also taken to process the exposures with filters. From the spectra obtained through the filters a fit of the lambda(X) function is performed. The procedure is performed for each setup (PRISM or GRISM, and SCI_PHOT or HIGH_SENS). The MIDI HIGH_SENS mode uses 2 windows on the detector (R1 and R2). The MIDI SCI_PHOT mode uses 4 windows (R1 to R4) We monitor here the stability of the dispersion by plotting the wavelength of a spectral channel given by the lambda(X) function computed by the reduction of the calibration data. The selected spectral channels corresponds to the expected central wavelength of the three narrow-band filters.