Need help to build rule for sending mail at certain times

Hello everybody,
I have several electricity meters that I can query via knx. For example, my main meter supplies two values (electricity supply and electricity purchase), each of which supplies the values to openhab (2.5.0) via a type number channel. Now I would like to send these values to several mail recipients at certain times (e.g. every first of the month - 00:00 a.m.). I have already installed the new mail-binding and configured an smtp server using a things file. I haven’t worked with rules yet, so I don’t know what a suitable rule should look like. Maybe someone from the community can give me a little help.

Excerpt from my knx.things:

Thing device Strom_eHz_netzbetreiber "eHz Netzbetreiber" @ "EG_Treppenhaus" [
    ] {
        Type number        : Strom_eHz_kWh_total           "Verbrauch in kWh: [%d kWh]"    [ ga="13.013:0/1/101" ]
        Type number        : Strom_eHz_kWh_total_Lieferung "Lieferung in kWh: [%d kWh]"    [ ga="13.013:0/1/185" ]

Excerpt from my strom.items:

Number    Strom_eHz_kWh_total	            "Verbrauch in kWh: [%d kWh]"     (VerbrElektro_Zentral)             { channel="knx:device:bridge:Strom_eHz_netzbetreiber:Strom_eHz_kWh_total" }
Number    Strom_eHz_kWh_total_Lieferung	    "Lieferung in kWh: [%d kWh]"     (VerbrElektro_Zentral)             { channel="knx:device:bridge:Strom_eHz_netzbetreiber:Strom_eHz_kWh_total_Lieferung" }

In Basic Ui it looks like this:
And finally the mail configuration in my mail.things:

Thing mail:smtp:mailserver [ hostname="", sender=""]

I would be very grateful for your help.
Thanks in advance.

Kind regards