Short answer: No.

In Jain Dharma, upvas (fasting) is a day of strict saiyam (self-restraint) and purity. One must observe complete brahmacharya (celibacy) for the whole period—this means no sexual activity of any kind, including masturbation. Even indulging in sexual thoughts, talks, or stimuli is to be avoided. This protects mental purity (bhava shuddhi) and prevents himsa that arises from passions.

Both traditions agree:

• Shwetambar and Digambar: During upvas/poshadha, a shravak/shravika tries to live like a sadhu for the day. Hence, any sexual act or self-stimulation breaks the vow of brahmacharya for that observance.

If you slip, do sincere pratikraman, reaffirm the vow, and strengthen restraint through:

• svadhyaya (scripture study), samayik/dhyan (meditation), quiet company, modesty in dress/speech, and avoiding triggers (erotic media, idle touch, late-night wakefulness).

This preserves the true spirit of upvas: reducing passions, increasing equanimity, and moving toward inner purity.