If your period started during the day on the 29th of Elul, the onah beinonit falls during the day on the 29th of Tishrei.
Day onot begin at sunrise and end at sunset. Night onot begin at sunset and end at sunrise.
Some couples have the custom to observe an additional onat perishah before or after the primary onah. There are two different customs: the 24-hour onah beinonit and the veset of the Or Zarua:
24-Hour Onah Beinonit
The basic requirement of the thirtieth day, the onah beinonit, is to observe the same onah (either day or night) as the beginning of the previous period. Thus, if bleeding started on the 29th of Elul during the day, then the onah beinonit falls on the 29th of Tishrei during the day. But, because the rules of the onah beinonit are somewhat more stringent than those of other vestot, some couples have a custom to observe the onah beinonit for the entire 24-hour period of the Hebrew date of the onah beinonit.
If this is your custom, and your period started during the day on the 29th of Elul, then you would separate for 24 hours on the 29th of Tishrei, both Friday night and Shabbat day. If your previous period began on the 29th of Elul at night, then you would also separate on both Friday night and Shabbat day. In either case, you would observe the entire 24-hour period of 29 Tishrei, from sunset to sunset.
Veset Of The Or Zarua
Some couples have a custom to observe the onah preceding any veset as an additional onat perishah (time of separation). This practice is known as the veset of the Or Zarua, after the medieval rabbinic authority who advocated it.
If this is your custom, and your period started during the day on the 29th of Elul, then the primary requirements of onah beinonit and yom hachodesh would fall on Shabbat day 29 Tishrei, and you would also separate on Friday night which is the preceding onah. If, however, your previous period began on the 29th of Elul at night, then the primary requirements of onah beinonit& and yom hachodesh would fall on Friday night 29 Tishrei, and you would also separate on Friday 28 Tishrei during the day, which is the preceding onah.