Why new and full moons bring the biggest tides
Tides are caused by the gravitational pull of the Moon and the Sun. When the Moon is new or full, the three bodies are nearly in a straight line, so the Sun's pull reinforces the Moon's pull instead of partly cancelling it. That combined pull raises high tides higher and lowers low tides lower, producing what are called spring tides [2][3]. The term 'spring' here refers to the tide 'springing up,' not to the season.
Because the Moon passes through new and full phases about every two weeks, spring tides recur on a roughly semi-lunar schedule. One study of ground-nesting shorebirds measured the spring tide cycle as about 14.75 days long, which is the interval between successive new or full moons [3].
When spring tides get extra high: the Moon's closest approach
The Moon's orbit around Earth is elliptical, so its distance varies. When a spring tide coincides with perigee—the Moon's closest point to Earth—the tide can be noticeably higher than an ordinary spring tide. A study of Indian coastal tide gauges found that spring tides during lunar perigee produced higher maximum tides than spring tides during apogee (the Moon's farthest point). For example, at Mumbai the difference in maximum tidal height between a perigean spring tide in January and an apogean one in August was about 87 cm, while at Bhavnagar the predicted difference exceeded 1 m [1].
These perigean spring tides can raise water high enough to flood low-lying coastal areas, and the study notes that this risk along northern parts of the Indian coast could grow with sea-level rise [1]. The same paper reports similar perigee–apogee contrasts for spring tides in March and April 1980, so the pattern is not a one-off [1].
Rare alignments that make spring tides even more extreme
Occasionally a spring tide coincides not only with perigee but also with perihelion—the time when Earth is closest to the Sun, around late December to early January. One researcher proposed that such combined alignments, which he suggested recur at roughly 3.8-year intervals, could be linked to lemming population cycles through a chain involving radon gas and plant chemistry [2]. That proposed link is a hypothesis, not an established cause, but it illustrates how the timing of spring tides can be pushed to an extreme when several astronomical cycles line up.
For the ordinary reader, the practical answer stays simple: look for the new moon or full moon on a calendar. If you want the highest spring tides—useful for knowing when coastal flooding is most likely—also check whether the Moon is near perigee, since that is when spring tides tend to run highest [1][2].
Sources used in this answer
Perigean spring tides along the Indian coast
Spring tides during lunar perigee produced higher maximum tides than those during apogee along the Indian coast, with differences up to about 87 cm at Mumbai and over 1 m in predicted tides at Bhavnagar.
Radon as a possible link between peak spring tides and lemming cycles
Spring tides occur at new or full Moon, and peak spring tides can coincide with perihelion and perigee, a combination the author hypothesizes may link to lemming population cycles via radon.
Nest initiation and flooding in response to season and semi-lunar spring tides in a ground-nesting shorebird
Spring tides occur around full and new moon, with a semi-lunar spring tide cycle of about 14.75 days.
The origin of neap–spring tidal cycles
The term 'spring tides' is usually reserved for tides related to the phases of the moon, distinguishing them from diurnal spring and neap tides.
Spring‐neap tidal effects on satellite ocean color observations in the Bohai Sea, Yellow Sea, and East China Sea
Spring-neap tidal effects vary within a lunar cycle in the Bohai, Yellow, and East China Seas, affecting ocean color observations.