Valmiki Ramayana Bala Kanda Sarga 14 Shloka 1

अथ after that, संवत्सरे when one year's time, पूर्णे (सति) was completed, तस्मिन् when that, तुरङ्गमे sacrificial horse, प्राप्ते (सति) had returned, सरय्वा: of river Sarayu, उत्तरे तीरे on the northern bank, राज्ञ: यज्ञश्च sacrifice by the king, अभ्यवर्तत commenced.

After completion of one year when the sacrificial horse had returned, the sacrifice by the king (Dasaratha) commenced on the northern bank of river Sarayu. Dasaratha performs aswamedhayaga on the bank of Sarayu according to vedic rites