New magnetic parameters, dAE and dAO, are introduced to determine the preferential times for substorm onsets. dAE and dAO are time derivatives of the standard AE and AO indices. By analyzing long series of dAE and dAO we reveal their seasonal and diurnal variations, which, unlike those of the original AE and AO, show reasonable similarity. The preferential periods of substorm onsets are shifted from the spring and fall equinoxes; they occur around 0900--1800 UT for the winter months with maxima of occurrence in October-November and February-March. Substorms are rarer during summer months at all UT hours, which is in agreement with the results of Newell et al. <1996> that aurora are suppressed in sunlight. The substorm suppression during summer months corresponding to large total conductance of conjugate ionospheres is consistent with one of the potential substorm triggering mechanisms, the magnetosphere-ionosphere feedback instability.