VI. In cases of urgency other times are allowable for baptism.
Wherefore, as it is quite clear that these two seasons of which we have been speaking are the rightful ones for baptizing the chosen in Church, we admonish you, beloved, not to add other days to this observance. Because, although there are other festivals also to which much reverence is due in God's honour, yet we must rationally guard this principal and greatest sacrament as a deep mystery and not part of the ordinary routine: not, however, prohibiting the licence to succour those who are in danger by administering baptism to them at any time. For while we put off the vows of those who are not pressed by ill health and live in peaceful security to those two closely connected and cognate festivals, we do not at any time refuse this which is the only safeguard of true salvation to any one in peril of death, in the crisis of a siege, in the distress of persecution, in the terror of shipwreck.
Source: Letters (New Advent)