וְאִם־לֹ֨א תִשְׁמְע֜וּ אֵלַ֗י לְקַדֵּשׁ֙ אֶת־י֣וֹם הַשַּׁבָּ֔ת וּלְבִלְתִּ֣י ׀ שְׂאֵ֣ת מַשָּׂ֗א וּבֹ֛א בְּשַׁעֲרֵ֥י יְרוּשָׁלִַ֖ם בְּי֣וֹם הַשַּׁבָּ֑ת וְהִצַּ֧תִּי אֵ֣שׁ בִּשְׁעָרֶ֗יהָ וְאָֽכְלָ֛ה אַרְמְנ֥וֹת יְרוּשָׁלִַ֖ם וְלֹ֥א תִכְבֶּֽה׃ (פ)
Mas si no me oyereis para santificar el día del sábado, y para no traer carga ni meterla por las puertas de Jerusalem en día de sábado, yo haré encender fuego en sus puertas, y consumirá los palacios de Jerusalem, y no se apagará.
Shenei Luchot HaBerit
When we read in 31,16: לעשות את השבת, the extra word את suggests that we should add a little from the weekday to the Sabbath, i.e. commence the Sabbath before sunset. The words: לא תבערו אש, "Do not make a fire" in 35,3, also hint at the statement in the Talmud that wherever we find that the Sabbath is also being desecrated, we are liable to find a conflagration (Midrash Hagadol based on Jeremiah 17,27). Observance of the Sabbath on the other hand, will lead to the redemption and the rebuilding of the Holy Temple. G–d will then be as a wall of fire for Zion, as we know from Zachariah 2,9.