Bhashya
वसन्तो हिङ्कार, प्राथम्यात् ।ग्रीष्म प्रस्ताव इत्यादिपूर्ववत् ॥
In plain words
Spring is the hiṃkāra because it comes first. Summer is the prastāva, and the rest is to be understood as before.
Close to the Sanskrit
Spring is the hiṃkāra, from priority. Summer is the prastāva, and so forth as before.