Day of Alleluia is one where the saint is just three stichera and no Doxastikon in the menaion. The reason there is variety is because Day of Alleluia once upon a time was the default daily format except for days where the saint was Doxology rank and higher, being days when Theos Kyrios was sung. Around 1100, there were about 180 days that were this format (that is specified as DoA or not having a troparion in the Synaxarion). In the Old Believer calendar, there are still some listed in October, January, April, and July (those months being outside fasting periods).