Standard star fields are to be observed every evening and morning twilight that appear to be clear, where "clear" is determined by a quick visual inspection of the RASICAM image, of the TASCA image, or (!) of the sky itself.
If possible, for each twilight period, 3 standard star fields should be observed: one at low airmass (X<1.25), on at medium airmass (X=1.25-1.65), and one at high airmass (X=1.65-2.10). It is generally better to observe the high-airmass field first, since its airmass will be changing the most rapidly.
For half-nights, 1 or 2 fields -- preferably a low-airmass field and a high-airmass field -- should be observed just before switchover. (If only one field, a high-airmass field is preferred.)
For evening twilight, use the DES/standard/Yzirg scripts (which start with Y-band and end with g-band); for morning twilight or pre-switchover, use the DES/standard/grizY scripts (which start with g-band and end with Y-band). (For those rare nights when we also observe in u-band, please use the corresponding "DES/standard/Yzirgu" and "DES/standard/ugrizY" scripts.)