For the ordinary owners' meeting (junta ordinaria), the notice must arrive at least six days in advance, and they are calendar days. Using the Spanish Supreme Court's count, the six days start the day after the notice is received, so the meeting can be held on the seventh day at the earliest. If the notice was posted on the community notice board (tablón), it only takes effect three days later: add those first.

How it calculates

Six days' notice. "La citación para la Junta ordinaria anual se hará, cuando menos, con seis días de antelación" (the notice for the annual ordinary meeting must be given at least six days in advance), under article 16.3 of the Horizontal Property Act (Ley de Propiedad Horizontal, LPH).
Calendar days. In civil time limits non-working days are not excluded: Saturdays, Sundays and bank holidays count (art. 5.2 of the Civil Code).
From the day after receipt. The starting day is excluded (art. 5.1 of the Civil Code), and the starting day is the day the owner receives the notice, not the day you send it.
The meeting on the seventh day. In judgment 433/2000 the notice arrived on 20 March and the meeting was held on the 26th. The Supreme Court annulled the resolutions because the meeting should have been held on the 27th. That is the "safe date" the calculator gives you.
The notice board adds three days. If the notice could not be delivered to the owner's address or the flat, posting it on the notice board takes effect "en el plazo de tres días naturales" (after three calendar days), under LPH art. 9.1.h. The calculator adds them before counting the six.

The calculator also shows the count some provincial courts accept, such as Granada in 2020, which allowed the meeting on the sixth day. It is one day earlier than the Supreme Court's count: treat it only as a reference and keep to the safe date.

What it does not calculate

Extraordinary meetings. The law sets no number of days: the notice only has to be given as early as possible so that everyone can know about it. If the agenda includes the annual accounts or budget, treat it as an ordinary meeting.
A separate second call. If it was not set in the first notice, it needs three days' notice and must be held within the following eight calendar days (LPH art. 16.2).
Catalonia. The Catalan Civil Code requires eight calendar days' notice (art. 553-21.2), not six.

How to prove the date of receipt

The date that matters is the date of receipt, so you need to be able to prove it: registered letter with acknowledgement, burofax, hand delivery with a signature, or a system that records delivery. We explain it in detail, with the court cases, in How much notice for an owners' meeting in Spain and in Can you call an owners' meeting by WhatsApp or email?.

Sources

Ley 49/1960, de Propiedad Horizontal (in Spanish), arts. 9.1.h, 16.2 and 16.3.
Código Civil (in Spanish), art. 5.
Supreme Court judgment 433/2000 of 26 April (in Spanish).
Granada Provincial Court, Section 3, judgment 238/2020 (in Spanish).
Catalan Civil Code, book five (in Spanish), art. 553-21.

The calculator applies the law and the Supreme Court count published as of the update date. It is not a substitute for advice from a lawyer on a specific case.